As I know there are different kinds of ptsd with different symptoms
Yes normally a Truama like that will be memory loss flash backs nightmares panick attack etc
But I think that's how its 'usually' and most cases is but not all
So maybe in his case is losing good memories?
But I'm not sure it either, it may be a whole different thing and the author just didnt want to search that much
Okay to answer you question, sihoo lost his dad in fire at his house, where his father told him to get out of the house and save himself and his mom. Which gave him ptsd w amnesia. (there are many types of trauma or ptsd, which are differ from person and their past or situation they suffered from. In sihoo's case he was in college when he got ptsd w amnesia (where he forgot happy memories with his own parents ). He had this type of amnesia way before meeting ml. So he knew he'll end up forget all of his happy memories and person involved with it.
Also to highlighting on his amnesia, he has trauma of losing his dad before his eyes and staying alive leaving him behind, which made him forget all of the happy memories he had with his dad, which automatically involved his mom. And he forgot everything about his parents. His situation triggers amnesia of happy memories. (and it keeps on happening)
It was mentioned he had trauma-induced amnesia right? From what I read, it said that the traumatic experience would be forgotten but it would sometimes resurface. In Sihoo's case, he forgets the happy memories. Also, he had the accident even before meeting Yesung. Is this still the same disease or what?? I honestly wanna knoww