Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

I'm trying to convert a .mp4 file to play with dash.js with video, two audio track and two or more subtitle tracks. resulted files and mpd play fine with one subtitle only, as soon as i had a second subtitle track, it is unsynced.

OS is Linux Slackware 15.0
``` 
# MP4Box -version
MP4Box - GPAC version 2.0-revrelease
(c) 2000-2022 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io

Please cite our work in your research:
        GPAC Filters: https://doi.org/10.1145/3339825.3394929
        GPAC: https://doi.org/10.1145/1291233.1291452

GPAC Configuration: --prefix=/usr --mandir=/usr/man --disable-opt --enable-pic --enable-pulseaudio --cpu=x86_64 --extra-cflags=-O2 -fPIC --disable-wx --disable-amr
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_IPV6 GPAC_HAS_SSL GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_MAD GPAC_HAS_LIBA52 GPAC_HAS_JPEG GPAC_HAS_PNG GPAC_HAS_FFMPEG GPAC_HAS_JP2 GPAC_HAS_THEORA GPAC_HAS_VORBIS GPAC_HAS_LINUX_DVB
```

My MP4 source test.mp4. MP4Box - MP4Box -info test.mp4 :

```
[isom/avcc] Missing REXT profile signaling, patching.
[iso file] Found stts entry with sample_delta=0 - forbidden ! Fixing to 1 <repeated several times>

# Movie Info - 5 tracks - TimeScale 1000
Duration 01:49:49.590 (recomputed 01:49:49.589)
Fragmented: no
Progressive (moov before mdat)
Major Brand mp42 - version 512 - compatible brands: mp42 iso2 avc1 mp41
Created: GMT Fri Apr  1 12:33:24 2022

iTunes Info:
        title: Test
        tool: HandBrake 1.5.1 2022011100

# Track 1 Info - ID 1 - TimeScale 90000
Media Duration 01:49:49.583  (recomputed 01:49:49.666)
Track has 1 edits: track duration is 01:49:49.583
Track flags: Enabled In Movie
Media Info: Language "Undetermined (und)" - Type "vide:avc1" - 157992 samples
Visual Sample Entry Info: width=1280 height=720 (depth=24 bits)
Visual Track layout: x=0 y=0 width=1329 height=720
AVC/H264 Video - Visual Size 1280 x 720
        AVC Info: 1 SPS - 1 PPS - Profile High @ Level 3.1
        NAL Unit length bits: 32
        Pixel Aspect Ratio 27:26 - Indicated track size 1329 x 720
        Chroma format YUV 4:2:0 - Luma bit depth 8 - chroma bit depth 8
        SPS#1 hash: 6208DEE775C5F3D73DE353432F02A82428672143
        PPS#1 hash: 388BF4B817C1896AC1198BB370180C1101833A4F
        RFC6381 Codec Parameters: avc1.64001F
        Average GOP length: 115 samples
        Max sample duration: 3754 / 90000

# Track 2 Info - ID 2 - TimeScale 48000
Media Duration 01:49:49.568  (recomputed 01:49:49.589)
Track has 1 edits: track duration is 01:49:49.568
Track flags: Enabled In Movie
Media Info: Language "French (fra)" - Type "soun:mp4a" - 308887 samples
2 UDTA types: 
        name: Stereo
        titl:  unknown type (13 bytes)
Alternate Group ID 1
MPEG-4 Audio AAC LC (AOT=2 implicit) - 2 Channel(s) - SampleRate 48000
        RFC6381 Codec Parameters: mp4a.40.2
        All samples are sync
        Max sample duration: 1024 / 48000

# Track 3 Info - ID 3 - TimeScale 48000
Media Duration 01:49:49.589  (recomputed 01:49:49.610)
Track has 1 edits: track duration is 01:49:49.589
Track flags: Disabled In Movie
Media Info: Language "English (eng)" - Type "soun:mp4a" - 308888 samples
2 UDTA types: 
        name: Stereo
        titl:  unknown type (13 bytes)
Alternate Group ID 1
MPEG-4 Audio AAC LC (AOT=2 implicit) - 2 Channel(s) - SampleRate 48000
        RFC6381 Codec Parameters: mp4a.40.2
        All samples are sync
        Max sample duration: 1024 / 48000

# Track 4 Info - ID 4 - TimeScale 90000
Media Duration 01:41:38.933  (recomputed 01:41:38.933)
Track has 1 edits: track duration is 01:41:38.933
Track flags: Enabled In Movie
Media Info: Language "French (fra)" - Type "sbtl:tx3g" - 550 samples
2 UDTA types: 
        name: Forced
        titl:  unknown type (13 bytes)
Alternate Group ID 3
QT/3GPP subtitle
        Size 1329 x 108 - Translation X=0 Y=0 - Layer 0
        RFC6381 Codec Parameters: tx3g
        All samples are sync
        Max sample duration: 77000760 / 90000

# Track 5 Info - ID 5 - TimeScale 90000
Media Duration 01:49:47.839  (recomputed 01:49:47.839)
Track has 1 edits: track duration is 01:49:47.839
Track flags: Disabled In Movie
Media Info: Language "French (fra)" - Type "sbtl:tx3g" - 3001 samples
2 UDTA types: 
        name: French
        titl:  unknown type (13 bytes)
Alternate Group ID 3
QT/3GPP subtitle
        Size 1329 x 108 - Translation X=0 Y=0 - Layer 0
        RFC6381 Codec Parameters: tx3g
        All samples are sync
        Max sample duration: 43821360 / 90000

```
Extracting subtitles in srt with MP4Box:

MP4Box -srt 4 test.mp4
MP4Box -srt 5 test.mp4
```
[iso file] Found stts entry with sample_delta=0 - forbidden ! Fixing to 1 <repeated several times>
Conversion done 
```

At this point if i add only one subtitle, the finale mpd file play fine and subtitle are in sync.

Adding WebVTT to test.mp4:
```
MP4Box -add test_4_text.srt:FMT=VTT:lang=fr+test_5_text.srt:FMT=VTT:lang=fr test.mp4
[isom/avcc] Missing REXT profile signaling, patching.
[iso file] Found stts entry with sample_delta=0 - forbidden ! Fixing to 1 <repeated several times>
Track Importing WebVTT - Text track 1329 x 720 layer 0
Track Importing WebVTT - Text track 1329 x 720 layer 0
Saving test.mp4: 0.500 secs Interleaving
```

Create mpd and files:

```
$ MP4Box -dash 10000 -profile dashavc264:onDemand test.mp4#trackID=1:id=video test.mp4#trackID=2:id=fre test.mp4#trackID=3:id=eng test.mp4#trackID=6:id=fr_forced:role=forced-subtitle test.mp4#trackID=7:id=fr_full:role=subtitle -out test.mpd
[Dasher] No template assigned, using $File$_dash$FS$$Number$

[MP4Mux] PID T7 ID 7 Sample 139 with DTS 295337 less than previous sample DTS 300000, adjusting prev sample duration <repeated several times>

[Dasher] Adaptation sets in period are of unequal duration min 6098.93 max 6589.59 seconds
[MPD] Generating MPD at time 2022-04-12T13:13:27.083Z
[Dasher] End of Period 
[Dasher] End of MPD (no more active streams)
```

MP4Box don't output the messages below with one subtitle track
```
[MP4Mux] PID T7 ID 7 Sample 139 with DTS 295337 less than previous sample DTS 300000, adjusting prev sample duration
[MP4Mux] PID T7 ID 7 Sample 141 with DTS 300709 less than previous sample DTS 314663, adjusting prev sample duration
[MP4Mux] PID T7 ID 7 Sample 143 with DTS 313921 less than previous sample DTS 333954, adjusting prev sample duration
[MP4Mux] PID T7 ID 7 Sample 145 with DTS 320083 less than previous sample DTS 350033, adjusting prev sample duration
``` 

Can't figure out what i'm doing wrong, is the source file not properly encoded or i'm missing some command argument ?


Adding a second subtitle track generate error messages
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Adding a second subtitle track generate error messages
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

I'm trying to convert a .mp4 file to play with dash.js with video, two audio track and two or more subtitle tracks. resulted files and mpd play fine with one subtitle only, as soon as i had a second subtitle track, it is unsynced.

OS is Linux Slackware 15.0
``` 
# MP4Box -version
MP4Box - GPAC version 2.0-revrelease
(c) 2000-2022 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io

Please cite our work in your research:
        GPAC Filters: https://doi.org/10.1145/3339825.3394929
        GPAC: https://doi.org/10.1145/1291233.1291452

GPAC Configuration: --prefix=/usr --mandir=/usr/man --disable-opt --enable-pic --enable-pulseaudio --cpu=x86_64 --extra-cflags=-O2 -fPIC --disable-wx --disable-amr
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_IPV6 GPAC_HAS_SSL GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_MAD GPAC_HAS_LIBA52 GPAC_HAS_JPEG GPAC_HAS_PNG GPAC_HAS_FFMPEG GPAC_HAS_JP2 GPAC_HAS_THEORA GPAC_HAS_VORBIS GPAC_HAS_LINUX_DVB
```

My MP4 source test.mp4. MP4Box - MP4Box -info test.mp4 :

```
[isom/avcc] Missing REXT profile signaling, patching.
[iso file] Found stts entry with sample_delta=0 - forbidden ! Fixing to 1 <repeated several times>

# Movie Info - 5 tracks - TimeScale 1000
Duration 01:49:49.590 (recomputed 01:49:49.589)
Fragmented: no
Progressive (moov before mdat)
Major Brand mp42 - version 512 - compatible brands: mp42 iso2 avc1 mp41
Created: GMT Fri Apr  1 12:33:24 2022

iTunes Info:
        title: Test
        tool: HandBrake 1.5.1 2022011100

# Track 1 Info - ID 1 - TimeScale 90000
Media Duration 01:49:49.583  (recomputed 01:49:49.666)
Track has 1 edits: track duration is 01:49:49.583
Track flags: Enabled In Movie
Media Info: Language "Undetermined (und)" - Type "vide:avc1" - 157992 samples
Visual Sample Entry Info: width=1280 height=720 (depth=24 bits)
Visual Track layout: x=0 y=0 width=1329 height=720
AVC/H264 Video - Visual Size 1280 x 720
        AVC Info: 1 SPS - 1 PPS - Profile High @ Level 3.1
        NAL Unit length bits: 32
        Pixel Aspect Ratio 27:26 - Indicated track size 1329 x 720
        Chroma format YUV 4:2:0 - Luma bit depth 8 - chroma bit depth 8
        SPS#1 hash: 6208DEE775C5F3D73DE353432F02A82428672143
        PPS#1 hash: 388BF4B817C1896AC1198BB370180C1101833A4F
        RFC6381 Codec Parameters: avc1.64001F
        Average GOP length: 115 samples
        Max sample duration: 3754 / 90000

# Track 2 Info - ID 2 - TimeScale 48000
Media Duration 01:49:49.568  (recomputed 01:49:49.589)
Track has 1 edits: track duration is 01:49:49.568
Track flags: Enabled In Movie
Media Info: Language "French (fra)" - Type "soun:mp4a" - 308887 samples
2 UDTA types: 
        name: Stereo
        titl:  unknown type (13 bytes)
Alternate Group ID 1
MPEG-4 Audio AAC LC (AOT=2 implicit) - 2 Channel(s) - SampleRate 48000
        RFC6381 Codec Parameters: mp4a.40.2
        All samples are sync
        Max sample duration: 1024 / 48000

# Track 3 Info - ID 3 - TimeScale 48000
Media Duration 01:49:49.589  (recomputed 01:49:49.610)
Track has 1 edits: track duration is 01:49:49.589
Track flags: Disabled In Movie
Media Info: Language "English (eng)" - Type "soun:mp4a" - 308888 samples
2 UDTA types: 
        name: Stereo
        titl:  unknown type (13 bytes)
Alternate Group ID 1
MPEG-4 Audio AAC LC (AOT=2 implicit) - 2 Channel(s) - SampleRate 48000
        RFC6381 Codec Parameters: mp4a.40.2
        All samples are sync
        Max sample duration: 1024 / 48000

# Track 4 Info - ID 4 - TimeScale 90000
Media Duration 01:41:38.933  (recomputed 01:41:38.933)
Track has 1 edits: track duration is 01:41:38.933
Track flags: Enabled In Movie
Media Info: Language "French (fra)" - Type "sbtl:tx3g" - 550 samples
2 UDTA types: 
        name: Forced
        titl:  unknown type (13 bytes)
Alternate Group ID 3
QT/3GPP subtitle
        Size 1329 x 108 - Translation X=0 Y=0 - Layer 0
        RFC6381 Codec Parameters: tx3g
        All samples are sync
        Max sample duration: 77000760 / 90000

# Track 5 Info - ID 5 - TimeScale 90000
Media Duration 01:49:47.839  (recomputed 01:49:47.839)
Track has 1 edits: track duration is 01:49:47.839
Track flags: Disabled In Movie
Media Info: Language "French (fra)" - Type "sbtl:tx3g" - 3001 samples
2 UDTA types: 
        name: French
        titl:  unknown type (13 bytes)
Alternate Group ID 3
QT/3GPP subtitle
        Size 1329 x 108 - Translation X=0 Y=0 - Layer 0
        RFC6381 Codec Parameters: tx3g
        All samples are sync
        Max sample duration: 43821360 / 90000

```
Extracting subtitles in srt with MP4Box:

MP4Box -srt 4 test.mp4
MP4Box -srt 5 test.mp4
```
[iso file] Found stts entry with sample_delta=0 - forbidden ! Fixing to 1 <repeated several times>
Conversion done 
```

At this point if i add only one subtitle, the finale mpd file play fine and subtitle are in sync.

Adding WebVTT to test.mp4:
```
MP4Box -add test_4_text.srt:FMT=VTT:lang=fr+test_5_text.srt:FMT=VTT:lang=fr test.mp4
[isom/avcc] Missing REXT profile signaling, patching.
[iso file] Found stts entry with sample_delta=0 - forbidden ! Fixing to 1 <repeated several times>
Track Importing WebVTT - Text track 1329 x 720 layer 0
Track Importing WebVTT - Text track 1329 x 720 layer 0
Saving test.mp4: 0.500 secs Interleaving
```

Create mpd and files:

```
$ MP4Box -dash 10000 -profile dashavc264:onDemand test.mp4#trackID=1:id=video test.mp4#trackID=2:id=fre test.mp4#trackID=3:id=eng test.mp4#trackID=6:id=fr_forced:role=forced-subtitle test.mp4#trackID=7:id=fr_full:role=subtitle -out test.mpd
[Dasher] No template assigned, using $File$_dash$FS$$Number$

[MP4Mux] PID T7 ID 7 Sample 139 with DTS 295337 less than previous sample DTS 300000, adjusting prev sample duration <repeated several times>

[Dasher] Adaptation sets in period are of unequal duration min 6098.93 max 6589.59 seconds
[MPD] Generating MPD at time 2022-04-12T13:13:27.083Z
[Dasher] End of Period 
[Dasher] End of MPD (no more active streams)
```

MP4Box don't output the messages below with one subtitle track
```
[MP4Mux] PID T7 ID 7 Sample 139 with DTS 295337 less than previous sample DTS 300000, adjusting prev sample duration
[MP4Mux] PID T7 ID 7 Sample 141 with DTS 300709 less than previous sample DTS 314663, adjusting prev sample duration
[MP4Mux] PID T7 ID 7 Sample 143 with DTS 313921 less than previous sample DTS 333954, adjusting prev sample duration
[MP4Mux] PID T7 ID 7 Sample 145 with DTS 320083 less than previous sample DTS 350033, adjusting prev sample duration
``` 

Can't figure out what i'm doing wrong, is the source file not properly encoded or i'm missing some command argument ?


--- end body ---
