If I enter multiple artist fields in the track list (using the undocumented \\ trick - I know no other way), I get:
Fine. If I then make an edit there, Mp3tag combines the artists into a single field:
This also happens if I just enter edit mode and exit with no change.
It would be good if this could be fixed.
Workaround: Upon each edit, manually resplit the artists list by replacing each ", " between artists by "\\". Or don't do edits in the track list.