I am seeing the same problem.
The difference is that I imported MP3 from My Music (originally transfered from iTunes on Mac).
That used to work.
The problem happens when skipping MP3 track to another track.
The important thing is that playback itself is successful, but skipping causes app crash.
Also, this problem is reproducible not only with MP3 but also AAC imported from My Music (AAC may not be officially supported, but it used to work).
iOS 12.4.1, App 18.104.22.1687
The problem was fixed in version 22.214.171.1249