I recently upgraded to Big Sur from Mojave and most everything went well. However, on the photos app, I have some folders that had only Smart Folders in them and they are all missing. Has anyone else had this problem?
I had the problem, but I'm not sure there's a fix.
When I first installed BS, Photos would "lose" my smart folders. After a few days, BS started throwing kernel panics and it got so bad I had to reinstall Catalina then BS. After the reinstall, I no longer had the problem with BS losing the Photos folders.
Have you tried to repair your Photos library ?
You can do that by holding down both the ALT key and CMD key and then (assuming you have Photos in Dock)
click the Photos icon.