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.
Thank you so much for the fix!