There's literally a similar manga where the guy gives up his tomboy potential girlfriend to the fucboi that appears in a similar fashion like this one.
And she clearly set up his flag by actually acting fucking cute in front of him, which is something she doesn't routinely do and usually only in front of our MC. But letting herself be like this with him was the inevitable out of character presentation to lure in the fucboi and give us the drama.
This whole arc is either going to break this manga, or simply severely reduce its likeability
i believe it will go all fine and is just tro ramp up drama for us.
Probably to make MC realize he doesn't like when FMC is away from him with someone else.
Doubt it would be something like him trusting his girlfriend as they aren't going out.
If it filters some readers...pity for them, is not that kind of manga.
If any the transfer student wil ldo all he can to avoid that if he notices the two energy when together, clear hints the stolen girlfriend is simply that a pal of his had a girl and she betrayed the other guy, and he feels guilty simply for being attractive.