I am using Audacity to convert M4A files to MP3, with two fields - 'DiscNumber and AlbumArtist - added to the ouput file's metadata which is exported as ID3v2.3. However, when the MP3 file's metadata is edited with Mp3Tag e.g. to remove duplicates, and the file is then imported into Audacity again, the additional metadata fields are now labelled by Audacity as 'Part of a Set' and 'Band'. This seems to happen only with Audacity as the tags are shown correctly labelled by other audio players e.g. foobar2000.
This may be a problem with Audacity rather than Mp3Tag but Mp3Tag seems to alter the additional metadata fields in a way that confuses Audacity.