Whilst playing through Carnival 2, I noticed during the first crescendo of the mission (where you descend the elevator into the obscura den and fall into a trap) there’s usually a bunch of fog, but in this instance there was not enough fog to impair visibility. Nobody else in the game reported it being perfectly visible, and this was in a Hi-Intensity Sniper Gauntlet (Ventilation Purge) mission.
Unknown. May be related to fog mission modifier.
PC - Steam