Found the fix!!
(Well, it fixed the issue for me: 2020 MS LR+; I understand that some older MCUs may have different/additional issues)
I discovered, largely by accident with a few "test" thumb drives: whatever drive you have the "TeslaCam" folder in the root directory *must* be FAT32, as we know... and it will ignore any music on that same drive.
I put an additional thumb drive in (FAT32), the USB icon promptly appeared, and it read the music off that stick just fine. Now suspicious, I temporarily renamed the dash cam folder to "_TeslaCam"--the dash cam went away, and the USB icon appeared, with normal access to all music on that same drive (the exact stuff it "couldn't see" after the 2020.12.5 update).
Played around with a few options. Turns out it's happy as long as there are separate partitions/volumes involved. I'm now using the same physical thumb drive I had been using, re-partitioned to have a ~40GB volume for the music (and, it's perfectly happy with the exFAT format for the music, so I left it like that) and a ~210GB volume for the dash cam (FAT32 required).
Annoying (and pointless?) issue, but--not a terribly painful workaround, and should only have to be done once.
Hope this helps!