The entries below started with a mistake, on my part.
I'm leaving them in, because they may help people figure what's happening,
especially with multi-value entries in artist(s) field(s).
The ending point is the opposite of where I started.
It seems that MusicBee is the cleanest with mp3 tags (at least ID3v2.3),
at that's a non-trivial issue, especially for those doing many imports & exports.
My thanks to "frankz" & "psychoadept" for working with me on this.
It might be useful, even for just this one issue (multi-value artist entries), to
do a comparison across many apps, to help with the import/export stuff.
I just ran a quick test and it seems that VLC uses spaces, where MediaMonkey
uses forward slashes, where MP3Tag uses double back-slashes, which MusicBee
interprets correctly and displays as a multi-value field.
Nice work.
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If I enter "Artists: Artist" as: Prince; AC/DC; other.
Then it shows as that.
And in the selection panes at the top, there are 3 entries.
(Working like Genres separated by semicolons & using multipe Genre selection panes).
(same applies to "Artist" multi-field)
But if I edit the same file in another program, and re-import (re-scan) in MusicBee,
then the semi-colons are changed to forward slashes, and the other options crap out.
Obviously, MB understands semicolons, and works fine with them.
But it HAS to STOP changing semcolons to slashes on import & re-import of MP3s.
I've tried the options/tagging, like using MP3v2.4 instead, but none of the options I found changed this behaviour.
How can I get MB to leave the semicolons alone?