This has happened to me twice in the past couple months.
Most recently, it was on SAN-BUF connecting in CVG. The flight out of SAN was a Delta 757. We did not get SSSS.
At the airport, I could not access the PNR via the kiosk. The agent who looked at me like I was a gross incompetent because I couldn't use the kiosk did not have any better luck even though he tried three times. We both got the same message that the PNR did not exist.
Minus the smirk, the agent went to the computer at the podium and found the reservation. Still had the same seats.
At this point, I'm settling for it being just another Delta IT screw-up. Next time it shows up, I'll try the change seat routine.
Art