I found out that if you write some of the TXXX tags by clicking on their field inside their column (e.g. mapped as %artists%), Mp3Tag writes it as TXXX:artists instead of TXXX:ARTISTS, which causes problems with other taggers such as MusicBrainz Picard. However if you tag your files by clicking on "Extended Tags...", the tag is being written correctly as TXXX:ARTISTS. I tested %artists%, %albumartists% and %album_artists% so far and all of them are affected, so it's possible that it's a bug with all TXXX tags. I'm using ID3v2.4 UTF-8 setting.
Actually, if a program can cope with user-defined fields (only very few can), then it should not bother about the case.
On the other hand: MP3tag allows you to adapt so many settings so that the target player is satisfied, you probably have to adapt your files so that the other program can deal with it.
Just for the record:
There is an open issue at MusicBrainz for their Picard related to this bug report:
"Read & write the ID3 tag fields in all caps": https://tickets.metabrainz.org/browse/PICARD-1101
I know I can modify fields myself, but shouldn't this be fixed as well? What I mean is that if I write ARTIST in Extended Tags, it gets saved into TPE1 tag. If I write it into the %artist% column, it gets saved into TPE1 tag (not tpe1). So why if I write ARTISTS in Extended Tags, it gets saved into TXXX:ARTISTS tag but if I write it into %artists% column, it gets saved as TXXX:artists and not TXXX:ARTISTS?
I opened that issue and because I was told that musicbrainz fields won't ever be capitalized, the remaining ones that I know of are TXXX:Writer and TXXX:albumartists. As for why it was decided that they should be all caps, I don't know.
I think because this is a user-defined field for which the user has full control.
The fields that find an equivalent in the id3 standard are mapped, user-defined fields cannot be mapped to anything but themselves.