I'll reply to yours out of the many that quoted my post since I think replying to yours would cover a lot of ground. Apologies in advance if this becomes a long read.
Basically what I meant is that the relationship between MC and Jaye is based on many misunderstandings which then lead to conflicts because they weren't communicating with each properly (this is both ways and not just MC or Jaye not communicating to the other). So for Jaye to act negatively toward MC is because from Jaye's point of view, MC didn't seem to care about her or what she wants.
Let's have a recap of all the things that MC did if looked from Jaye's point of view:
1. MC suddenly avoided her (because MC didn't want her to teased by her friends) and from Jaye's pov, her brother/bestfriend just suddenly shut her out for absolutely no reason.
2. MC read her diary (invasion of privacy, and the present-MC acknowledged that there were things in that diary that she really really really wanted to be kept private)
3. When she fought with Christian after MC went home from the championships, and MC saw him getting out of the room, MC had accusing look and he didn't want to hear any explanation from her. Think about it this way, you just fought with your boyfriend because you didn't want to have sex with him, then you went out of the room, saw the guy you actually love looking at you with anger/hatred and didn't want to listen to you. This is a time when you needed the support, but you were once again abandoned.
4. MC then decided to announce it to the whole family, friends, and guests of the party that you (Jaye) were almost raped. Think about it, rape is a really hard issue where there are TONS of rape victim that couldn't go to the police because how hard it is to talk about it. Then for someone else (let alone the person you love) to announce it to a group of people just like that? While yes it was a rape attempt and not a successful one, it was still extremely embarrassing.
5. Then MC disappeared the next day. Completely shutting down the communication between them again.
6. Fast forward to parents' deaths, MC wasn't there to support her. This is not necessarily MC's fault, but it was again a miscommunication due to the phone number being blocked (from years ago that Jaye might or might not have done?). Once George confirmed that MC was indeed there, Jaye's reaction was to ask MC why he didn't try to contact her. She wanted to reconnect, but MC decided that it wasn't the right time and didn't make the contact.
7. The scene outside George's office. MC decided to leave on Uber while Jaye had to stop him to talk. Again, she wanted to talk.
8. Even after Tara had shown Jaye that MC's number was blocked (which Jaye said that she would never do so - no idea right or wrong in reality), MC had disappeared on his Uber. Notice the pattern here? MC's action is always just to run away instead of communicating to solve the conflict.
9. Fast forward to Aspen story, Jaye constantly texted MC to reconnect. She apologized about the blocked number and all that.
10. Fast forward to the flight back from Aspen, Jaye asked MC to get coffee together before meeting George. MC declined.
11. Outside of George's office, Mallory kissed MC in front of Jaye. From Jaye's pov "No time for coffee with me, but time to have kiss with this girl". This time MC was the one trying to explain for once, but Jaye was no longer just jealous, she just felt defeated because after all that (including the "read between the lines" text to MC), there was another girl that had MC instead.
12. This then turn into anger in George's office because apparently Mallory is not just a stranger but actually involved in this whole thing. With the kiss that they shared so openly in front of her, Jaye was basically on warpath to start hurting MC in return. That's why she started saying things like "fuckboy fund" or "Captain Starfucker"
13. (If Christian is the +1) it makes sense because Christian would be the one that hurt MC the most. It's not like Jaye is planning to marry him. Just to get him there to piss off MC in retaliation of the things that MC had done to piss her off. That's how it's justifiable.
Jaye's action is driven by all of the above, not by a single particular action of MC.
The key thing here is that miscommunication is how things go wrong between the two people that actually love each other. Both Jaye and MC had their fair share of mistakes, but a healthy communication between the two could've solved tons of misunderstandings that would end up with them hurting each other instead. So far, Chasing Sunset is a tragic love story.