This bug viewtopic.php?f=7&t=63297&p=1087742
Has returned at some point.
Short summary: FakeContrast lowers the brightnesss of some walls in low lightlevel sectors to the point where they become pitch black, and stop being affected by the lightspot that supposed to illuminate the area around the player slightly on Doom, Software and Vanilla lightmodes.
Here's gallery example at E1M3 after picking up blue key: https://imgur.com/a/m1fK26B