This is due to a change in v2.89 for the internal mapping of MusicBrainz IDs
- [2018-06-04] CHG: added mappings for special fields with MusicBrainz IDs for ID3v2 and MP4. (#18405)
With this change, I've added an mapping from internal (written to the tag) to external (displayed in Mp3tag) representation of the MusicBrainz ID fields. This was necessary, because other applications (e.g., MusicBrainz Picard) or libraries (e.g., TagLib) are using different names for those identifiers depending on the tag format. An example for those different internal representations is
MusicBrainz Artist Id
This has the drawback that if you configure the Tag Panel to write
MusicBrainz Artist Id (as in your example above), it would work find when tagging MP3s but would also write a field
MusicBrainz Artist Id if you decide to tag FLAC files (where instead
MUSICBRAINZ_ARTISTID would be correct).
What I came up with is a set of standardized field names for MusicBrainz IDs that are written in the correct way depending on the tag format used. Those names are
If you use any of those as field name on the Tag Panel, everything should work as expected.