Happened to me with no warning on the LAS-ATL red-eye last night. At the 24-hour check-in window (am gold elite), changed from 4F to 27A, which the seatmap clearly showed as the back exit row. Made the selection & got the "willing & able" dialog box u get when selecting exit rows. Had no reason to think anything was wrong.

Well, in boarding and getting to the seat, it was just a normal cramped seat. The FA said they can't try to reseat back to the real exit seat (30A), because the flight was full.

Plane was an A321neo. Sound familiar?

Needless to say, I got no communication from Frontier about it. I wondered if paid upgrades to that row would be treated any better, or would have to get to their seat and then be surprised as well.

Yeah, looks like an IT issue they very much need to fix.