Hi psychoadept,
It's strange - I'm seeing it in MP3tag, it shows as 2 backward slashes eg. 'Alternative\\New Wave' - however it does seem to show the semi-colon in the windows tag inspector.
What's even stranger is that when I correct the tag in MP3tag, it works on Poweramp, even though it remains the same to the Windows Explorer tag.
In MusicBee tag inspector the genres are each displayed on a different line, eg. Genre: Alternative / Genre: New Wave
Cheers