"Flagged" emails are not marked as "flagged."

I'm revisiting a discussion from a year ago. Using the latest Big Sur, I flag an email -- and the flag does not appear next to the email header. Nor does it "disappear" into a flagged box on the sidebar. I would be happy with at least getting flagged emails separated, but my emails just sit there, unfledged, unmoved? Has anyone discovered a remedy for this? I have also tried going into "view" and enabling the message filter. I still get nothing.