BUG: JSON serialization with orient split fails roundtrip with MultiIndex
- Closes #50456
- xFail for mark split deprecated.
- All [code checks passed].
- Added an entry in the latest `doc/source/whatsnew/v2.0.0.rst`

Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.