Spoiler coming so if you don’t want to know, don’t read.
The reason he punched him is because lanshan xie is in another persons body. He died and his brain was put into the body of a criminal that looks just like him and that is why he gets those random flashes of killing people and why his mother doesn’t recognize him. In the scene where he was punched, luifei realised that the person kissing him wasn’t lanshan and so he punched him and then lanshan like switched back to himself, if that makes sense?
I can't find the one I read before so... here
https://jpmtl.com/books/3505/1921239/mobile
just starting to read this from the first chapter.
Lanshan’s ‘body’ is actually the criminals body. Lanshan brain is inside the criminals body and that’s why lanshan sometimes does things that he wouldn’t normally do. Lanshans actual body is probably buried in the other guys grave. This links back to the start where they were talking about lanshan having no scars even tho most agents who go undercover would have scars. He has no scars because he isn’t in his own body
So they did the deed.
*brain fart* so why’d he punch him in chapter 48