I'm glad to learn of your successful experiments with the UltraStable films. The fact that these (presensitized) color pigment films were made more than 15 years ago, and are still usable, illustrates another reason why dichromate
(which would have spontaneously hardened these gelatin emulsions in less than 48 hrs) was abandoned as a sensitizing agent.

My guess is that the stain you are seeing is a consequence of (a) washing out the exposed pigment film onto a gelatin coated paper (here, dye-transfer); (b) not clearing the print as described in the lab Manual (which unfortunately you do not have); and (c), as you said, the mordant may be playing a part (did you thoroughly wash the DT paper before using?).

I'm pretty sure the clearing baths will not remove sensitizer stain after this much post-processing time, but if you wish to try, here's a snip from the (soon to be posted) manual relevant to that protocol:

6. Clearing

When all four colors have been processed and fully dried, residual
chemicals are removed by immersing the print in Clearing Bath A for
1 minute, followed by a brief cold water rinse and then placed in
Clearing Bath B for an additional minute. After a final rinse, the
print is dried and ready for finishing.

Stock Clearing Bath A:
Water (at 110 F) 930 ml
Potassium Permanganate 6 g
Sodium Chloride (Table Salt) 14 g

Stock Clearing Bath B:
Water (at 110 F) 940 ml
Sodium (Meta)Bisulfite 30 g
Sodium Sulfite 30 g

To make working solutions of A and B, dilute each in cold water at 1:20.