Responses

I mean it was kinda there since the beginning that they knew each other when they were kids, the scar on his head, the scent having an effect on him, the insomnia, the memory lost that's connected to the insomnia, all signs points to an earlychildhood accident. They probably only met once but that one meeting had an impact on chaejun, made him feel safe or something. And it probably wasn't that memorable for seunghyun, just something that happened.
Is such meme material
If they knew each other as kids, that would be an annoying and unnecessary trope to put in such a refreshing story. :/