I wonder who would be more jealous then: Jill or Josy. Josy seems much worse in this regard.
You won't do something like this while being a really jealous person. At the point of game shown above, Jill can be barely considered as Josy's friend - they know each other for a few weeks, but she is a direct competitor in fight for MCs heart. But Josy is still friendly and quite supportive with her, even when MC is in Throuple at the same time. Even though she's literally "dying" inside. True jealous person will end up starting a fight and/or a lot of arguing.
Josy's reaction and behaviour after EP9 in case of break up with Throuple is absolutely normal. According to how MC managed to hook J&M up and then dumped them as if nothing happened. Maya is more cold and reasonable, because she is used to failures in her life (and love life) and considered Throuple as an experiment, while Josy seemed it like that all is worked out and she can have both people she is in love with. She doesn't agressive in any way to any of MCs new girlfriends, she is just angry on him personally. Because his behaviour was like "all is ok, we're bonding", not a little hint that something is wrong, nor a little warning.
The worse thing you can do - is to lead someone too far, while pretending that all is going fine and then *poof" - let's break up. It's related to both, males and females. Be honest and speak up and talk, before it's too late. Such emotional damages in such age could be very devastative (literally). When you get older, you may even laugh on this, but it feels absolutely different in 18-20+ And when someone is judjing how stupid/not grown up characters are, remember yourself back in your 16-18+
If they all will act cold and indifferent, then they aren't 18yo kids, they are probably 30+ adults (in comparison, Bella being in her late 30s, acts pretty "natural" and cold, for example, even though she also got hurt if MC decides to end things with her). And such writing would break all the immersion and believability.