It turns out, if you register new TXXX:Whatever tags (mixed case) as custom tags in MusicBee, they will be read but always stored in uppercase by MusicBee in the MP3 file. When registering COMM:Whatever tags (mixed case) in MusicBee, MusicBee preserves the capitalization. Is this intended behaviour ?
I am asking because I'm tagging my files initially with Picard from MusicBrainz which tags with a lot of mixed case identifier versions like:
MusicBrainz Release Track Id
MusicBrainz Release Group Id
I then use MusicBee to download the lyrics and also use MusicBee to produce the SYLT variant of the lyrics.
Then I save the tags from MusicBee, but actually MusicBee UPPERCASEes the original MiXeDcAsEd versions coming from MusicBrainz like:
MUSICBRAINZ RELEASE TRACK ID
MUSICBRAINZ RELEASE GROUP ID
When I want to re-tag now with Picard, Picard doesn't recognize the uppercase versions of it's identifiers anymore.