On my first Nest mission with the released game, and areas I walk into don’t become illuminated upon my entry. There is no mist at play here. After a sprint, the new area 2-3 squares past my man (that I should now be able to see) is as dark as it was when it was 15 squares away at the beginning of the turn.
No new special creature or effect in play that I can see.
After playing a bit, the areas are only dark for 3 of the 4 camera rotations. Once I get to the ‘right’ camera orientation, the normal illuminated view I’d expect comes up. That feels buggy to me.