MC didn't tell the elf girl, that he wanted to continue at some point. And he bought a house (or did he rent it?) so she might have gotten the wrong idea. But did the thought really not occur to her, that such a OP adventurer might to go on. He even told here, that this town isn't his "real home". It's been only a month and she just simple assumed he wanted to settle down. And then, when he decided as their group leader, it is time to move on, she snapped. I know, that she formed different plans for her own including MC/that she grow to have different wishes, but the evidence, that he grow her to become a mage and level her up is there. For what reason did he do that? To flirt around? And then everyone tells the MC to apologies. He was insensitive to her feelings, but she wasn't about his?
With the right arguments in your favour, "You can easily win the argument, it's the aftermath that's always a pain in the ass." With some women, this is sadly true. There are always people you can't talk with in most situations. And it's a pain to end up in a relationship with such a person, that starts to punish you, because you don't share their POV.
Back on the topic: She even does not listen to his orders as a group leader, because she has her own ideas about the well being of the group. (Seeing her friends die or hurt badly, is to hard on her, even if she can resurrect or heal them later.) I'm sorry, but this kind of stuck up behaviour is dangerous.
But to be fair, it looks like she doesn't want to be an adventurer at all. As I see it, she wants to settle down with her prince charming. And this is what she might have asked him in one and a half years, "We know (are dating) each other for nearly two years. We should be asking our selves, what we want with our lives. (I want you to quit as an adventurer in the near future and to marry asap. Disagree and I will punish you and threat to leave.)" This is over-the-top and a sexistic picture of women, but this fictional elf girl and the world around her seems to be just like that.