It's quite simple, really.
Semicolons are used as multi-value separators for tags, but before that they were used as separators in common written language.
That's how I use them in the album tag. Not to separate different values that should be split apart, but as a separator for reading.
It occurs mostly with Classical music, but not only. For example, a cd that reissues two LP albums together, the album tag will include both album names separated by a semicolon.
And by default, MusicBee understands that. It doesn't split names with semicolons in the album tag (like it does with the Artist tag, for example).
The problem only occurs when I try to use the album tag to generate a virtual tag.
I hope it is clear now.