What'd she do wrong this time? Telling your partner not to go to a dating event is too far these days?
The thing is that Yamada didn't know it was a dating event. For her that was a party, I can understand the feeling of wanting to go to a party with friends, so I expect they'll discuss over that and Yamada will show Kase that she can be (at least a little) independat and wanted things, and that doesn't mean that she'll come back to another mixer xD but also she can try makeup and go to parties. Kase was reflecting her insecurities all this time in Yamada, I can understand why she just calls out and decide to go to a party (other thing is that that wasn't one).
Uh... NO! She knew it was a mixer! If you look at the chapter where Hana invited her to it (https://dynasty-scans.com/chapters/yamada_to_kase_san_ch04#27) you'll see that the event page she showed her on her phone said:
"Student's Only Mixer
City Party
A Party for those
looking for love"
Yamada knew EXACTLY what she was going to!
She NEVER thinks about the concept that people other than Kase might find her attractive, so she wouldn't think that her going to a mixer might result in some guy trying to get together with her. She only focused on how uncomfortable it would make her, NOT on how she was going to an event intended for people looking for sexual partners of the opposite sex.
Kase had her own issues here. Not telling Yamada about the trip she was going on, when she was going to be without her phone and unable to be in communication with her at all for a few days, until the day before, and apparently only telling her because her roommate mentioned it within earshot of the phone, was not good girlfriend behavior. Finding out that Kase's been making a habit of accidentally wearing her roommate's clothes and using her stuff just made the whole thing worse.
However, on the balance, Yamada was the one who was way off here. Kase has no choice about having a roommate and, frankly, about being absent minded and not being able to tell one pair of shorts from another, really. That just seems like something she'd do. Yamada should never have agreed to go to a dating event with her friend. She should have told her friend that she can't do that because she's dating someone.
The real solution to this whole problem is for Yamada to come out to her friend about who she is dating and for Kase to come out to her roommate about who she is dating, though I think that is already a done deal, since I'm sure the roommate has heard Yamada's voice over the phone and just heard Kase exclaim that they are dating.