After rereading the entire manga, and even reading the web novel, the most likely reason for their breakup was Aya's jealousy and Yuu's indecisiveness. He just couldn't move on from his first love, Hikari, but he also didn't let go of Aya. That makes sense for why they argued in the classroom. I think the conflict with Aya pushed his thoughts back to Hikari. He’s rethinking the big changes in her behavior and the time they spent together over summer break, which suggests that his feelings brought his feelings back full force. By chapter 21 Hikari was winning by a mile, and that kiss was a desperate move, kind of like what we saw at the station in chapter 25.
This is further reinforced by the extra chapter we're getting Comitia 150 on November 17, in which we will see Aya grow increasingly jealous of Hikari after seeing her and Yuu act like a natural couple compared to their relationship that relied heavily on sex. Overall, I think Hikari is winning in the end. Trust the Plan.
https://x.com/sa_ra_na_mi/status/1853747699092537643 
