Sorry for taking your time, I had tried multiple full rescans and reinstalls to no avail, but I figured something out so I'll share my solution here:
I used MP3Tag to copy paste the tags while removing the ID3v1 tags
More info here: https://community.mp3tag.de/t/removing-id3v1-tags/4043/11
So my only conclusion is that Poweramp was reading the v1 tags for some reason- removing them from the trouble files and ensuring they only have v2.3 tags solved all my issues!
Note that Musicbee has the setting "Also create ID3v1 tags" on by default, so I recommend turning this off!
Hope this is helpful for devs (to find a way to default to v2.3 tags) or for other people having this issue!!