In my experience, Plex has always been very reliable in this regard when configured to use embedded tags. The first thing to do is make sure you’ve done everything to tell Plex to use embedded tags (the checkbox in the Library Advanced settings is only part of it).
If that’s all good, I would look at the tags in Mediamonkey. The Performer Sort Order (TSOP) ID3 tag is defined in the ID3 v. 2.4 specification. It does not exist in the more widely-used v. 2.3 spec. If Plex is actually reading the TSOP tag, I would suspect the following possibilities (in no particular order):
- The TSOP tags are not consistent throughout your audio files.
- Mediamonkey is set to write tags in ID3v2.3, which may lead Plex (and any such program) to ignore or unreliably read the TSOP tag (I don’t know how tags are written, or if Mediamonkey can be configured to do this - it’s a bit of a speculation).
- Some sort of corruption in the tag.
