Wowza, these comments. Real incel vibes. However, just based on ch1 I could say they'd be less upset if the MC had any fuckin' emotions at all. Like, if they had a good relationship they should probably have some sort of fuckin' response to her showing back up, but he doesn't seem to give any thought to it. Seeing as they dated for just a couple months it appears...that kind of makes sense to a degree. For most of us it'd be awkward to have an ex there, but if they just went out a few times over a couple months then they're basically fine.
The issue ultimately is...I doubt they don't have a hefty backstory, which puts me on the side of uncomfortable with how nonchalant he is. It'd be fine if he just let them have a room, but the interactions like they're family is just so bleh. The tone of the story is more the problem than anything else.