
I get where you're coming from but I dont think this is one of those... if your ex cheats on you, obviously they're gonna be the first to stop loving you and you are the last to love them, but eventually that love will fade, ya know. And as we saw, that love has already faded.
It would be more alarming if Ash already stopped loving the ex before the ex cheated because then....why were they even dating to begin with? Was Ash just playing too? You see how it can create other problems regarding Ash's own personality if you expect Ash to be the one to not be in love first.

Mistake ?? Dude cheated because he wanted to AND ran away. Cheating can never be a mistake.
And i found it even more reassuring that he hates him for that and looks extremely angry (as he should) cause you know your man doesn't put with this shit and will definitely not cheat on him.
And in another way thanks to the ex cheating our baby boys are now together.

If im understanding what you're saying is that ash not loving hazel anymore because of something he did, not because he fell out of love with him as a person, wouldnt make you feel good, correct? Because it's not hazel as a person, but an action.
But i would digress and say that they're one in the same. Your choices as a person create who you are and your character. Your behaviors and actions are what make you who you are.
So while ash did fall in love with hazel because of his character and actions, it's also what made him fall out of love because hazel decided to do something that is fundamentally frowned upon. And then to top it off, he used ash like an Air BNB whenever he needed to.
It would be more alarming if ash still loved hazel despite the betrayal and tried go separate this action as a one-time mistake than completely cut him off.
Because then it's like tryna separate the art from the artist, but some things are just unforgiveable.

Looks to me you wanted Ash to dump Hazel, not the other way around? And it kind of upsets you that Ash was not Lyle's first serious relationship. And how Ash could have remained in the serious relationship, if not for the cheating.
But that's just the thing. You view the cheating as a "mistake" while Ash views it as a relationship ending behavior.
I think you're projecting a little how you expect Ash to handle the situation? How Ash introduced Hazel was how their relationship ended and it says nothing about Ash's current feelings regarding Hazel anymore. It was about their past. It's not really an issue to tell your new partner your ex cheated, that's why you moved on.

In regards to how ash introduced hazel to lyle, i believe it's more of a way to put lyle's mind at ease. He's essentially establishing to both lyle and hazel "yes, this is someone i dated in the past, but i no longer am associated with because we ended on terms that were unacceptable."
It was also a way for lyle to get the gist of it AND for hazel to realize he really screwed up.
After that, with ash saying that nobody knows why they broke up and he won't tell anyone (except lyle), he has nothing to be sorry for because they're strangers, that's the biggest way a person can say "i feel nothing for you anymore."
Ash towards hazel is like a gust of wind. You don't really even acknowledge it.
He doesnt care what happens to hazel, has no ill feelings towards hazel, doesnt acknowledge hazel, ash is completely and totally indifferent towards hazel.
In that sense, it's to tell lyle that he has nothing to worry about this relic of the past because it holds no significance in his life anymore, in a negative or positive manner. There's nothing. He doesn't think about hazel at all, doesn't reminisce, nothing.
In ash's actions from this chapter, i believe it's to really solidify to lyle that hazel means nothing to him anymore.
Sure they're alpha and omega, but that biologically symbiotic relationship of their secondary genders means nothing to him because he has lyle, even though they're an alpha/alpha pair.
To lyle, he really is afraid of not being enough because of the abuse from his grandpa, so it's really important for ash to establish to lyle that the secondary gender thing is not important. Even without a bond, they're meant for each other.

to me cheating is unforgivable and not just a mistake, let me be clear about that. I understand that Ash wanted to reassure Lyle of where he was standing with thim, but yeah, like I said before I'm projecting in some way (is it even 100% possible to read a story without doing that?), and I would be like "so you're saying you're with me because your ex is trash?"
When he could had just said "I used to love him, but not anymore, you're the person I love now because of who you are"
errr... I don't know if I'd feel good hearing that the person I'm dating mainly hates their ex because they cheated on them... I mean so it's just because they made a mistake? not because you actually stopped loving them? I don't know...