Maybe im forgetting something but what did Yuu actually do wrong here? Date a different girl at some point?
For that matter does ayami even know that he's the dude Hikari is into?
Ok, so for a long time when Hikari was talking about Yuu to Ayami and her two friends she was referring to him as Ta-kun because his last name is Takamura. In chapter 11 when Hikari and Yuu are on the amusement park date there's a group chat going on and the blond friend makes a pun based on Yuu's name which causes Ayami to say "Yuu? Huh? Isn't Hikari's boyfriend's name Ta-kun?"
This is the moment that Ayami starts to put 2 and 2 together and realizes that the guy she dated and ghosted and the guy that her best friend is in love with are the same guy. You see later in chapter 13.5 where Ayami confirms with Hikari that her boyfriend's name is Yuu to which Hikari says "H-He's not...He's not my boyfriend yet." In Ayami's internal monologue she confirms that it was the amusement park date that started the realization. She goes on to think "God...this isn't fair at all." People in these threads have been arguing because it's not 100% revealed yet if Ayami still loves Yuu and hasn't told Hikari because she wants him back or if she values her friendship with Hikari so much she doesn't want to reveal the truth because she's afraid of destroying her relationship with Hikari.
As far as the answer to your "What did Yuu actually do wrong here?"...it's complicated. There's currently two sides to the debate in these threads. I am vastly oversimplifying them below.
1) Yuu is at fault because you should never lie. His lying has hurt Hikari and made things worse than him just telling the truth.
2) Yuu is not at fault for not mentioning he dated Ayami as he has no reason to reveal his past relationships in detail to a prospective future girlfriend.
There's more to those two sides than that but I already type too much to begin with and I don't need to start a freaking novel explaining the nuances of both sides.
I also want to point out that so far there has been no indication that Yuu knows Ayami and Hikari know each other. Until we get Yuu's POV (if that even happens) we won't know 100% what's truly going on.