She acted irrationally BECAUSE she was feeling hurt. She's literally been gaslighted by her ex-boyfriend before. Like, I also found her actions in this chapter frustrating, but it's anything but hard to understand. It's so easy to go "Chanu's so nice and sweet though :(" as a reader, but she is living it. And considering what she's been through it makes sense that she doesn't just trust him implicitly. The fact that he was super cagey and vague about his real feelings was put into (the wrong, but still) perspective by the whole bet thing.
Personally I agree she has every right to be upset, and my "defense" (if you will) of Chanu isn't necessarily that he is nice and should be forgiven. I believe consequences to his actions are integral for their growth as a couple. I just don't necessarily agree that the bursting out into tears and ending it all immediately felt right for this characters. Felt a little lazy as a writer, because I personally felt no satisfaction by her finding out and leaving him. I didn't feel like he got his comeuppance (I just felt he was completely ignored when he didn't deserve that) and she pulled a full coward out of no where. I just feel like there was a better way to write this, and is less of a direct critique on her and more the writing...but I digress lol
That bit about comeuppance opened my eyes a bit. I totally get where you're coming from now. Personally I appreciate the low stakes drama that forces her to stand up for herself, because it's so clear that she'll forgive him and that he'll learn to take things a little less lightly.
I mainly read romance fantasy and BL, where the drama is so much worse and often gives me a headache, so I've been enjoying this little personal growth arc. But you've made me realise that it is a little... contrived? I don't think that's the right word though, because the set-up happened so long ago, and honestly it still feels like pretty grounded drama to me.
It's really nice to have a civil "agree to disagree moment" like this, thank you for your nice reply :)

I believe there is a world where she can be upset and justified, and people can also be disappointed in how she handled it. I will admit her immediate drop of the hat response was very jarring as a reader, considering how calm and mature she had handled the entire situation before this. It felt...odd. I understand her feeling betrayed and hurt, and wanting space, but is no one going to give him the benefit of the doubt? He has changed more than she has...idk. I know why she is upset, but I can't help but look at her reaction as one-dimensional in the space of the whole narrative.