Pretty sure that even though the sex was good, Yumi heavily dislikes Senpai as a person, which is why in the flashbacks, we always see her not interested at all in being with senpai, and once she figures out who Haru is and starts approaching him, we don't see her again with senpai. Senpai scenes are pretty much all delusions or IF content.(But Senpai's Also Her Best Sex Partner So Maybe It's Not Roleplay???)
The timing was certainly WTF lol, but I think Haru decided to propose to Yumi specifically because she said that she'd love him no matter what, so even though it is an awkward scene, I don't think it's that un-earned. It's Haru making a decision and comitting to it.No clue what NT00 plans to do next but the romantic proposal scene felt un-earned tbh
Idk, I just find it funny more than anything.