Sometimes (let's say for 1 out of 200 records), when I tag files in Picard, the existing genre tag "Drum & Bass" gets added at the end. For example,

"Drum & Bass" -> "Drum & Bass/Drum & Bass"

"Drum & Bass/Jungle" -> "Drum & Bass/Jungle/Drum & Bass"

This seems to happen only with Drum & Bass. I assume the "&" might create a problem sometimes. I think this has only happened to MP3 files for me, but I'm not sure. I tag a lot of drum & bass records, so it might not actually be exclusive to that, but I've tagged enough that I would've noticed otherwise. Not that this doesn't show up like that in Picard. I can only fix it by deleting the tag in foobar2000 and then re-adding the correct tag. Afterwards, it works. So the way foobar2000 tags might play a role, too. The vast majority of MP3 files I tag use id3v2.3 exclusively, with a few id3v2.4s in there.

Note that this bug has existed for many years. Unfortunately I don't have a affected file right now.