@gr8g
I believe its a psychological thing. Ch. 236 is the flashback and goes over it, Suteguma sees the faces of people he knows he can dominate completely. The way I interpreted it, Suteguma was never able to see faces of adults as a child as a means to cope with his situation; the people didn't matter, just whether they were adults or children. He never considered anyone equal to him, so he couldn't connect emotionally with anyone. Once he realizes that has been living a pitiful existence and gets his strength back, he sees the face of his abuser clearly for the first time, and kills him. When he leaves the sewers he realizes he can see some faces but not others. (As an aside he was never able to fully parse Robert K's face when he was sneering at him either)
Somehow Suteguma could
see the faces of people he can "win" against, so he can pick his battles effectively and never taking a losing gamble or engagement. Which is why when Usogui faked as if he fell into Suteguma's trap, Suteguma revealed that he was suddenly able to see Baku's face. Of course, Baku was faking, so when Suteguma enters the incorrect number, he realizes he saw something he should never have been able to see (Baku's face).
We aren't given a concrete reason for it, but that page explains what the ability is and puts forward the idea that its a product of Suteguma's abused childhood.