Night Marsh lighting suddenly stops working, causing much darker environment than normal

This one is a little harder to explain, so I've included a longer video to demonstrate the before and after. Details below, and also in the YouTube video description:

Part 1: Before problem began, unlit areas are rendered as very dark (normal)

Part 2: A subset of environmental lighting suddenly stops working

Part 3: Effect persists through the remainder of the run, with everything much darker than usual