I realised this just now when I tagged some files with MusicBrainz Picard and then tried to use another websource using data from MusicBrainz.
Apparently the tag field mappings used by Mp3tag are a little out of date compared to what MusicBrainz uses now.
Files tagged with Picard look like this:
Currently it's kind of a pain to work with both Picard and Mp3tag (I have to use Picard for the scanning from time to time which is why I never noticed before)
Picard's Mappings can be found here