No one comments about it, really???

no its for effect, his eyes arent drawn too and its because he's turning around. its meant to be suspenseful and creepy because you know this creep showed up again and theres a cliff hanger

His eyes is hidden by the glasses, it's ok, his mouth is just missing. It looks bizarre and almost comical.

Yeah it’s literally intentional to make it bizarre and creepy. They even made sure the glasses look transparent with no glint, and u can see that they traced the wrinkles so u can see them under the glasses but not the eyes. Manhwa authors draw faces without the full features all the time for the suspense effect

Any examples? Because I've never seen a clear, not shadowed face with no mouth like that... To me it looks like an error.

I don’t remember off the top of my head right now, but just about any suspense, thriller, or horror manhwa has had something similar. I think “Home Sweet Home” might’ve had a lot of those facial features omitting moments. If you look up titles with those tags you should be able to find examples of it. Or just on google/ Reddit.

Yeah, found only one example in Tokyo Ghoul, a specific character drawn often with no mouth to express that he is stoic and suppresses his emotions, not at all what we see here. Also no mention of this 'no mouth' art choice being something which even exists. BUT found that omitting a mouth often happens because author makes a mistake while drawing ╮(￣▽￣)╭