When I get stuck, looking at the overview of all functions for MusicBee formulas often gives me some fresh idea.
Funny enough, that's the thread I ran to when I saw your solution.
At first, I couldn't understand the purpose of your $Len in there. So, I'd ran to your cheatsheet thinking there was an explanation I'd missed regarding that function.
I edited my previous post to make it make more sense.
When I looked at this problem (as with all others), I hadn't come from an angle of "first find
exactly how many genres there are and bingo" since I've never figured out how I would do that.
So thanks to you, going forward, I now know how to get the exact number of occurrences in a tag. Cheers!