I don't know but I wished hyunsoo remembered their anniversy and wanted to make a big surprise...and... for that reason he pretended to not remember:))
Maybe I'm trippin but he said hyunsoo has been worried about his mom for a while and I thought it means he forgot their anniversary because his mind was somewhere else.:'(
it says it in ch.11 when he returns home after staying out he says “im the one who ended up ruining our anniversary morning” ,, so hyunsoo didnt forget
I don't know but I wished hyunsoo remembered their anniversy and wanted to make a big surprise...and... for that reason he pretended to not remember:))