If you look int he fare rules at booking, I've seen many of the 'discount' fares that we call -UP now saying that it is a first class fare and not economy with instant upgrade.

Fyi this almost happened to me flying out of LAS yesterday after the convention. Irop on my inbound flight so would miss my connection, bunch of mess with auto rebooking, then checked with the app and a new option through ATL came up to get home all in first (was already up front with a RUC before). Selected it, then noticed my ticket showed first but with last row in coach seat assigned. Had to call and wait on a 30 min hold on the DM line to get an agent to fix it. Luckily the system had that last F seat tied up even though it put me in the back of the bus.