I've been seeing similar abnormalities and have come to the conclusion that MusicBee is fine
with ID3v1 and ID3v2.3 being written & updated (by itself or other programs) .. BUT has no
option for specifying which to show. I'd love to have a simple switch in MB for that.
The only workaround I've found is to do all my scans, downloads, edits, then do a full re-scan
in MB, followed by full search-and-replaces (of all the things I already did that to, of course).
So, for example, "artist 1/{f}/artist 2" is what appears in MB, but looking at the MP3 file, I find
that it HAS stored my edit to "artist 1; {f}; artist" BUT only in the ID3v2.3 tag at the end of the file
(the first is at the top, per ID3v1 format).
Maybe there's a way to automatically sync tags .. or to chose which to show