ID3v2.4 defines for TMCL (Musician credits list) and TIPL (Involved people list) that every odd field is a role (instrument, vocal role or function) and every even is an artist or a comma delimited list of artists.
Sometimes one or more performers in the list are lacking a role like in this recording
Picard 2.6.3 moves them to a TXXX:performer custom field. I still have some older mp3-files where those performers had been appended by Picard to the TMCL tag. This lead to the funny result that odd field performers appear as instrument/role of even field performers.
I guess that there are good reasons why performers without role are not just written as is, like for example:
vocals; Johnny Cash; guitar; Johnny Cash; bass; Marshall Grant; drums (drum set); W.S. Holland; ; June Carter Cash
The TXXX:performer tag does not seem to be documented in Appendix B: Tag Mapping — MusicBrainz Picard v2.6.3 documentation and I was not able to find a ticket on that subject. Is this sort of a “beta version” feature and subject for changes?