Man, this whole thing is just shit for everyone involved, isn't it.
Tsubame clearly does like Ishigami, but also can't actually let herself fall in *love* with him because she's leaving.
Ishigami does not want anything if it's not in love and long term, he doesn't just want a quick month romance that is forgotten.
And Iino is just generally hurting as she's been for the entire manga.
Feelings, man. They'll kill ya.