Originally Posted by
anti_ice
Yep, MG is bang on. But I thought they fixed this bug. I saw it a year or ago, but I am pretty sure I recently saw United domestic front cabin showing up as biz as it should. "First" yeah, ok!!

It shows this way if the only segments you book are on UA or US metal in the domestic/transborder cabin. However, if any AC J space is involved, this problem arises.
(This is not just an SE problem. There are a couple of UA transcons on internationally configured 3-cabin aircraft that cannot be booked in F even though there's an F cabin. In fact, if you query F, the flights don't even show up. However, plug in a search for C (or Y), and they pop up. Its also the reason if you book C overseas space on UA as an award, any domestic or transborder space will only be made available in Y.