There's a bug where if a pawn goes to a tile where there's 0% Unnatural Darkness, their faces "glow" under the dark, texture-wise I believe it's under 'Head_Blank', specifically 'normal_east' and 'normal_south'. Code-wise..? I have no clue, if I had the modding smarts, I would've taken a crack at it myself.
https://imgur.com/a/YuBM4vJ