So this was 2-3 years ago if he is a high school senior now.
Which i believe is too much time to make them so lukewarm more than being awkward.
We don't know for sure.
There's a weird mistranslation or misunderstanding about the time here.
First, they apparently started dating at Middle School, not sure if mentioned when.
Second, apparently the break up was in the graduation so their final year at Middle school (I don't remember this but someone mentioned it, and it makes sense with how they were using the same uniform in the flashback.)
Third, it gets mentioned they are highschool seniors when they start living together as siblings.
And fourth, various dialogues mentioned they broke up less than a month ago from the current chapters in the present.
...so then how are things really? Did they actually dated into their second year of highschool?