A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 100,368
It sounds like the GA didn't realize that you were upgraded and awaiting a FC seat assignment, not just a coach passenger without a seat (perhaps with a basic economy fare if they're offered in this market). You should have asked the GA instead of just boarding and taking 25E, especially as a DM.
Check the DL app and delta.dumb to see where DL thinks you were seated on the flight and what your class flown was, then call and complain.