@Yamibi
It definitely smells of the old misunderstanding as a plot contrivance so MC and Mai-san can be pulled apart, and Koga given the spotlight.
Granted, Mai has been shown to be moody, petty, and quick to get upset in the previous series, but after all that she and Sakuta have been through her reaction here feels like a regression just for the sake of keeping the story going. That said, perhaps this will only be a temporary thing and she'll be back within a few chapters to redeem herself, rather than my fear that it will be used for milking as much drama a possible from the current situation.
While I do agree that there are observable differences in culture and social interactions in western and eastern cultures, it isn't enough to assuage my misgivings about the direction the story is going. I could just as easily argue that:
1) Pausing and giving thought to your words as opposed to responding on impulse comes down to the individual rather than where they live. Even if you are correct that it is more prevalent in East Asian countries, that doesn't mean that it is necessarily rare in western ones.
2) Mai's reaction, if it can be considered reserved, is still an impulsive, emotional jab at Sakuta for something that wasn't his fault. Looking at both her words AND body language, she clearly thinks he was doing something lewd with Koga willingly, and doesn't stop to give him the benefit of the doubt. Even if she doesn't approach it logically (thinking maybe Koga is the aggressor here because she is on-top of him), she clearly has feeling for Sakuta after all they've been through together. That all goes out the window, and her first thought is "HE'S TWO-TIMING WITH A YOUNGER GIRL?!" Why assume the worst instead of, like you said, pausing to think and understand the situation as it really is.
It is not impossible to believe that she could act this way based on what has been established about her character so far; however, at best it makes her come across as an immature brat who will make Sakuta grovel to fix her own misunderstanding (not a healthy-sounding relationship to me), and at worst it looks like the author undoing most if not all the progress and growth she has made with Sakuta to justify focusing on Koga without having to create an alternate timeline or start a new series without Sakuta as the male lead. Either way, I think he deserves better after all he has gone through.