Ok, i took a look and it seems to be a bug in TagBox when choosing "append" as tagging behaviour. It is just strange that in some cases (depending of field) it works and in some not.
If the question still persists, switching the radio button in File > Preferences > Advanced > Tagging > Vorbis&Flac > Metadata writing mode or possibly even (un)checking File > Preferences > Advanced > Tagging > MP3 > ID3v2 rev. and quirks > Map TPE2 to Album Artist check box could help.
Regardless of the context, users can only edit actual metadata fields, not file-derived technical/format-specific information or other special fields. The latter are not open to alteration, for the simple reason that they’re supposed to be invariant due to being deduced from the stream (or path, or etc.) itself.