Actually, there's no amnesia, just lies.
That's exactly what the title explain: "Imitation Happy Life"
But it's not a lie. He has dissociative identity disorder and chooses to forget the bad things that happen to him.
Wasn’t amnesia - Someya was blocking out painful memories as a defense mechanism. This probably stemmed from a painful childhood, because its referenced multiple times that his parents didn’t really treat him right.
I think this was probably supposed to get another chapter originally... It feels like the author just rush finished it...
Also, the amnesia part doesn't make any sense... Amnesia doesn't work like that.