I'm not referring to the actual content of the tag, but the name of the tag itself.
In order to find out which program does what, I need to retag some albums again with several programs, and delete everything in between. It's a tedious job.
All my files are mp3 files with only ID3 v2.3 tags.
I have discovered that several files were tagged with multiple fields like i.e.:
MUSICBRAINZ ALBUM ID = edec8307-7668-4136-8068-f5361dfd85eb
as well as
MUSICBRAINZ_ALBUMID = edec8307-7668-4136-8068-f5361dfd85eb
Apparently, Picard wrote the latter in there, ignoring the fact that it was already there, just under a different name.