
I would be suspicious if I was a rich and wealthy person like Taekyung and found every trace of evidence that could possibly have been left behind by my lover wiped clean. Also if the guy who had been my lover had a high memory retention and somehow didn't remember me in any way, shape or form, it would be irrelevant whether or not I thought they considered our relationship unimportant or not. There's just NO way anyone like that could forget even a passing acquaintance. So nope Taekyung's the one with the problem.

That’s your opinion so I’m completely fine with that. I thought about this so many times myself and I came to the conclusion that if I was in this same position, I’d feel so hurt the moment my lover says he couldn’t remember me while being completely serious and innocent about it that I’d probably just run off and cry again lol and also knowing Heejae was smart, I would think he lied about not knowing who I was or just seriously wanted to forget who I was :/ but people are different so I think it’s totally fine for you to react differently.
I think many people believe the seme chose to trust the older brother over Heejae despite knowing how evil the brother was, but I think that’s not the case. The seme NEVER wanted to believe the older brother and that’s EXACTLY why he couldn’t let Heejae go after all these years because he knew something was off, but couldn’t be sure that his suspicions were right. So he carries on with his life, thinking that Heejae screwed him up albeit internally being suspicious of how everything ended. And then he gets the chance to meet Heejae again and gets hopeful that maybe Heejae can clear things up and tell him that it was all his brother’s fault they broke up, but guess what? Boom, the guy doesn’t even REMEMBER him. And that’s the worst case scenario right there.
What does that tell him? That his doubts were true all along, because if Heejae really was forced to break up with him because of his brother, then he would’ve still remembered him but nope because of the incident (and remember, the seme has absolutely no means to find this piece of information because the older brother covered everything up) Heejae doesn’t remember jack shit about him. He doesn’t need to ask Heejae anything anymore: the fact that Heejae doesn’t remember is MORE than enough evidence that everything was a lie. When someone forgets you, you don’t go thinking ‘oh maybe they got amnesia’. The ONLY logical explanation to this supposed amnesia is that the seme was never an important part of Heejae’s life, which is why it’s perfectly reasonable at this point for the seme to go crazy and take revenge.
Now, communication is key to solving this, but again, remember the plot. Even if the seme goes up to Heejae and asks “do you remember me”, Heejae will just say no and make everything ten times worst. Why? Because he isn’t supposed to tell people he lost his memories, became stupid, or whatnot because it’d put his family under the bus. And even if seme goes up and tells Heejae they used to date and all, Heejae would just freak out because he DOESNT want his memories back plus he can’t even confront his brother. And the seme can never know Heejae lost his memories unless it comes from Heejae or the secretary themselves which can’t happen easily.
So yeah, it’s frustrating as hell to us readers because we have third-party information that the seme can never have on his own. This problem is such a huge mess because each party involved has so much hidden agendas that MUST stay hidden and it’s so complicated =_=