I agree with
@Ingway
I found Tsubane's decision normal, distance relationships are difficult, not everyone can do it, especially if you just started a relationship and separates after. Does not mean that she does not like Ishigami, after all she did not said anything until now to us be sure, she may like him, but be afraid to take a relationship with him.
And then maybe she thought that having a physical relationship with him, without having to be something with commitment could be something to take advantage of the little time they still have together, does not mean that it could end with just once, it could happen more times and not just sex, and when they separate, both supposedly do not suffer much from it.
But Ishigami understood this proposal as if he were going to be "just one night", he thought she was doing it out of pity for him, and he was rude to her because of it. I can understand his displeasure, but in the end, it's a situation that we do not know the complete truth, and they need to talk.