Smart mailboxes, Flagged, VIP folders all stopped working in Big Sur

Spotlight Reindexing (https://support.apple.com/en-us/HT201716) finished. I rebooted (twice). Started up Mail, and everything is working again (for now).

Once you're in Spotlight > Privacy, click the plus sign "+" at the bottom left and you'll get a file selection pop-up. In that pop-up, I found it easiest to select my entire home directory. To do that, don't select anything but just click the Choose button and then click OK on the following "are you sure" pop-up. That should put your home folder into the list of locations that Spotlight is prevented from searching.

From there, I found I had to quit System Preferences and then reboot my Mac. Once back in, wait until Spotlight is finished reindexing. I was able to tell when it was finished by opening the Spotlight search box and typing a few letters. There then appeared a progress bar saying "indexing..." and I was able to watch that until it finished. Not too long, in my case.

When indexing is finished, I go back to System Preferences > Spotlight > Privacy, select my folder from the list and click the minus sign "-" to allow it to be indexed again. Finally, quit System Preferences and reboot one more time. After indexing is finished (as before), when I get back into Mail everything works correctly again - Smart Folders, search, etc.

And now it's November 22, just over a week later, and the same problem is back again.