The problem I had with that elevator scene before is that all troopers at this point should know about the infamous FN-2187.
The FO is MASSIVE (in terms of the amount of people), of course they wouldn’t know about Finn’s betrayal.
I know that, but knowing how these sorts of regimes work, I’d think they would broadcast all over the place how terrible this 2187 guy was. They would use it as an opportunity for propaganda and make the stormtroopers want to hate the guy.
Or maybe they’d keep it under wraps. Who knows.
EDIT: Proof that they should recognize him comes from TFA. During the battle of Takodana, he is recognized as a stormtrooper by another one even though there’s no real indicator of it. Remember when the guy with the baton thingy shouts “Traitor!” at him?