The head canon is saying she’s lashing out and being abusive.
Where?
Sh’e not “sad about her band breaking up” she lacks confidence entirely and she thinks she’s just lost the connection that brought her and Aya together in the first place. She’s scared she’s got nothing to offer Aya so she’s using the physical relationship they’ve built like a crutch and trying to avoid the root problem.
And people saying “they should just talk” as if these character flaws and insecurities weren’t established in the first chapters just makes me wonder what world they’re living in where flawed people make purely rational decisions.
Chapter 6 establishes Mitsuki thinks she’s boring and has nothing to offer Aya so why would she even call her. Nevermind that Aya doesn’t even know she not a guy.
Joe has her at a concert and says “what are you talking about, you have this”. The rock scene. That’s it. That’s not how Joe meant it but note that he doesn’t say anything about Mitsuki’s value, just that Mitsuki has rock.
We haven’t learned yet that Joe has the relationship skills of a walnut. So it seems like it’s encouragement.
Fast forward to after the timeskip and we watch Mitsuki fumble and then completely lose the one thing she believes makes her worthy of Aya. Poor self confidence is a bitch and if you think your value only exists in one thing it’s not going to be a simple matter of going “oh that made me sad”. Mitsuki panics, she tries to avoid it, she clings to things like their physical relationship, all the while believing she’s got nothing to offer anyone.
I don’t feel like this is super hard to follow. Like it may not be the most satisfying drama but it happens in real life and it was very clearly set up from the beginning.