En route, currently in ATL. DSM-ATL flown without incident. While enjoying lunch in the D Club, got message my seat on 717-200 ATL-LIT had been changed from 21C to 10B, seat I didn't want. Went to desk, first agent first saw me still in original seat, then another agent saw me in 10B. 13B also open, so asked if I could be moved there. She said no problem and gave me a boarding pass for correct flight in 13B. And now that flight is completely gone from app and website. What happened? Confirmed also with gate agent I'm on the flight and I am. Both he and the Club agents said "happens frequently." Used to fly DL a lot (PLT), since COVID a Gold and haven't ever run into this before. How common is it?