Responses
Someone asked and the author said it's because Dohyun's life is harsh and exhausting with nothing in his life that's dependable so because of stress he doesn't remember Sungho.
Here's the link for full Q&A chapter, it's in Chinese though:
https://m.dm5.com/m544423/
I'm guessing Dohyun doesn't remember Sungho because of psychological repression. Sungho affected and hurt him so much in the the past that his mind decided to forget that's why he ended up with represssed memories. MAYBE