I will concede, that is a little concerning. The only real explanation is if Ayami and Yuu broke up before the events of the manga, which is really up in the air at this point. Some people seem to believe that’s the case, some people believe that their breakup is a more recent development. It’s possible that Yuu never brought it up but that seems like a stretch.
Actually, I just looked through the chapters and it actually is kind of possible for Yuu not to have told Ayami about Hikari while they were dating. First off, Yuu seems to already know that Ayami is somewhat insecure when it comes to his childhood friend, so he already has a reason to not fully reveal all the details to Ayami. Secondly, he could always explaining reading manga with Hikari in his room/chilling in his room as just “hanging out with a friend” to Ayami if the topic ever arises. Secondly, it’s implied that the amusement park IS when Ayami finds out that Hikari’s crush is Yuu. She realizes this through text when she realizes that Yuu and Ta-kun are the same person. So, she actually would know about him being with her at the Amusement park and them going to the swimming pool together, since she and Hikari went swimsuit shopping together later on. So, if they did break up during Hikari’s arc, and not before, then it likely would’ve been after the trip to the amusement park but before the swimsuit shopping. The amusement park could even be a reason why they broke up. This is just one possibility though, who knows where this manga is going.