My headcannon/prediction for the twist that sets off the flashback sequence is that Kev arranged for Robert to get hassled so he could play hero to win him over. Extra points if Kev reveals this while they’re talking about how they met/got together and he doesn’t see what’s bad about that (or at least doesn’t see how the fact that time has passed and he and Rob are close doesn’t justify his actions). Bonus bonus points if Kev did this on multiple occasions, anytime he and Rob fought or he thought Rob was getting tired of him/thinking about breaking up with him. That would obviously be a huge betrayal for Robert given how bad they’ve implied he had it, to find out that even some of it was not only caused by but intentionally arranged by the one person he thought he could trust would obviously be devastating.
And the at would also explain the promo photo of Kev crying on the couch because Rob would obviously kickoff if not outright breakup with him, so he’s only now realizing how bad he messed up and that he might lose Rob forever.