This is just my running theory but maybe it's because Lemon's family or an acquaintance of hers wanted her to focus on her studies and get to a prestigious university and they saw that Lemon having a lover was being a hindrance in her studies so they told her to quit the romance but she doesn't want MC to not be able to move on and thought that making him hate her by lying that she cheated on him is the best way to make him move on.
This would explain things like how MC was surprised that Lemon was very smart in university despite them going out in highschool which I think he should know if Lemon was like top rank in the class. At the end of the day this is just an opinion formed by how typical Japanese romcoms like this usually goes but it kinda makes sense in a way. Anyways, at least we finally got another college romance where it's all adults (albeit young adults). Always hated highschool romcoms cause it makes me very uncomfortable with the fact that they're underaged. As sad as it is this series will probably end like how Nisekoi did, blondies win in the end lmao.
That's a pretty solid theory, wouldn't be surprised if the manga ended up following most of those beats. Although Lemon being surprised that the MC still remembers her lie is a bit strange, but I guess that can be handwaved as just her having high expectations for him. The initial premise still seems like ragebait, but I have to admit it's a pretty good hook.
Also ditto on the adult romcom part. So many WSJ romcoms are set in high school, they all start to become redundant after a while. Jump+ is atleast starting to explore having characters that are legal age in their romances, so hopefully we'll get more diverse plotlines in the future.
My theory, from that little scene about how lemon was a pushover in junior high; those 3 girls cornering Lemon with one girl sobbing- I think they pressured her to dump him since crying girl loved him soooo much and wanted a shot with him. So being a pushover caused her to lose what she valued most as the next panel cuts to them being broken up.
This might come up again when she realizes her BFF is also into the guy she loves, so she'll have to decide whether she's gonna step back and let someone else date him (again) or if she's gonna stop being a pushover and go after him.
(Now dumping ur bf bc someone else wants him is ~Incredibly~ stupid, but I've seen stupider manga logic)
Why'd she lie about it if she still loved him???? I feel like the tone of the manga is a bit off. It's too cute for it to be a manga you hate read, but some of the plot points are too stupid for it to be something you read for a cuteness cleanse