Are you saying <Artists: Artist> is not matching value ";" when it should?
Yes, that's true.
It's also not just isolated to highlighting rules as I originally thought. It occurs anywhere you can use Artist: [whatever] for sorting.
My rule
Artists: Guest / has a value
normally parses as Artists: Guest is not null, but that doesn't work
As a work-around I changed it to
Artists: Guest / is not / [blank]
which parses as Artists: Guest is not '' and that works
Couldn't even get <Artists: Artist> to work using regex match ^\;*