The issue for me surfaced some time ago when I was trying to tag my mp3 files with the "artists" tag. Picard writes the tag field as "TXXX:Artists", but some other taggers (namely Mp3Tag and Foobar2000) will write it in all caps as "TXXX:ARTISTS" and since ID3 tag names are case sensitive, it shows as another field among the taggers. We talked a little bit about this on irc the other day and I was told by @Sophist that he plans to convert (possibly all) the tags to upper case so "transcoders who don't know the Picard-specific tags can transcode them correctly".

As of now, there are tickets for TXXX:Work (PICARD-1049) and TXXX:Artists (PICARD-1416).