So basically, from what I remember (it's been a while since I read it), the couple was actually pretty healthy throughout the main story. Then the sequel completely changed that, and I honestly don't know why.
The MC and the ML have a conversation about their relationship and their past partners. The ML straight-up says he doesn't see a future with the MC and that their relationship will eventually end. He even talks about the MC's future partner like it's some inevitable thing.
Then he starts talking about his exes. They were all rich and successful, and he basically admits he dated them because it benefited him. There were barely any real feelings involved—it was mostly convenience and sex—so he's still friends with most of them because they were never actually in love.
I don't even know how to explain that whole conversation properly—you honestly have to read it yourself. It's not like he cheated or anything, so don't get me wrong. It was just such a huge shift from the ML we got to know in the main story that it genuinely left a disgusting feeling in my stomach. I remember reading the comments on Bato, and almost everyone felt the same way. People were genuinely disgusted by the way the ML was talking, and everyone kept asking, "What is happening to this couple?"
That was the end of Season 1. Then Season 2 introduces the third guy in the picture, who's apparently been in love with the ML his whole life. He comes in to cause problems, but that gets resolved pretty quickly.
Instead, we spend almost the entire season reading a flashback about the ML and this new guy while the MC and the ML's argument is basically put on hold. It was honestly frustrating.
Eventually, the ML goes to talk things out with the new guy, they resolve everything, the MC sees that the ML gets jealous for some reason, and realizes his feelings are deeper than he thought (that enough for mc to forgive), and from what I remember, they pretty much make up by sleeping together. It's been a while since I read it, so I might be remembering some details wrong, but that's the general idea.
The whole sequel just felt like unnecessary drama. I actually found myself rooting for MC to break up with the ML and end up with the new guy instead—which I never thought I'd say after reading the original story.

Tbh this really ruined the couple for me