Well in this case Seo in DIDNT know a single thing that happened back then, he feels overwhelmed and embarrassed by all his actions and thoughts, more like ghosting is just keeping a distance but still keeping an eye on him. Because seo in feels underserving of MCs love and seo in thinks being by his side will make him suffer. Selfish? Yes. But also MC never told him shit and kept it a secret for what? Embarrassment perhaps? Both of them didnt wanna talk about it (mc wanted to brush it off) and so the ghosting was out of an internal conflict more than just "i hurt u in the past so i ghost u now" doest that make sense?

i'll never understand the logic behind the "I hurt you in the past so I'll ghost you now" thoughts. you're just hurting him again..? and of course he was going to be targeted while you're away when will yeongwoon be happy!!