Seung-hyeon has said in a previous chapter that he does not remember most if not all of the younger kids he knew in his childhood. He definitely roped Chaejun into that too because he thought Chaejun was much younger because of how small he was. And Seung-hyeon was quite young himself, a developing brain can forget a lot of stuff as new memories get created. Plus with all the shit he went through from middle to highschool on top of the fact he felt unloved at home, it makes sense he would hardly remember a kid he never saw again as an elementary schooler.

Ok I kind of get why chaejun doesn’t remember but how could seonghyun not remember that?