Sorry but maybe this BUG report is in the wrong section.
The issue which started a few weeks ago is ...
Caves I had created by creating a trench and using Granite Rocks prefabs to make a roof, were pitch black inside now they are lighter with a "milky" fog effect.
This also occurs under the terrain where I have used the 'holes' tool in the Terrain Tool to gain player access.
If you disable FogHaze_Default_1 by ticking the 'Disabled' tick box in its 'Object Properties' the problem disappears.
I checked Eden Everon and it has the same problem so it is not an issue with my terrain.