Exactly seems people didn't see the red flags Inseo has always had because he was really good at hiding them for the sake of not scaring Yihyeok away so he always kept his distance we don't know if Inseo had some unhealthy habits to cope with restraining his possessiveness, but now that he is allowed to express he is letting everything out it's hitting Yihyeok really hard e.e
Right? Inseo is a red flags from the start Inseo always tried everything to get Yihyeok's attention. If Yihyeok liked or admired something, he would immediately doing it (like playing a piano and motorcycle thing) so that Yihyeok would pay attention to him only. When Yihyeok became friends with Tak and Sanho at school, Inseo wasn't happy about it, but he joined in anyway because he didn't want Yihyeok to hate him again like they did when they were younger. I think the author is emphasising how Yihyeok will respond to Inseo's possessiveness in this side story.
In several comments I read they asked why Inseo had changed. But guys, Inseo has always been like that, even when they were young. In fact, Inseo's mother explained to him when he was a child that it's normal Yihyeok had other friends. And because Yihyeok had rejected him once when they were young, all this time, Inseo had suppressed his possessive side and his feeling. At the end of the chapter, Yihyeok said that he accepts Inseo as he is, even though Inseo had already told Yihyeok that he is possessive. This is why in this side story Inseo no longer hides how possessive and anxious he gets when Yihyeok is close to other people. He is so freakin annoying since day 1