Hi Hevedy,
As I previously mentioned this is not a bug. Just settings that need to be set in the Atmospheric Fog.
With the Landscape Mountains, these settings are setup in a very specific way to tweak the look of the level. They have been manipulated within their given functions.
The documentation here is a good resource to start with: https://docs.unrealengine.com/latest/INT/Engine/Actors/FogEffects/AtmosphericFog/
Specifically the settings in the Landscape Mountains that can be adjusted to alleviate the issue is:
- Sun Multiplier
- Distance Scale
- Altitude Scale
Altitude Scale specifically controls the Z scaling for the falloff of fog in the distance. This means that below a certain point the fog is going to be much thicker.
You should take some time and get to know these settings here in the level and just try adjusting the value of different settings to see what they do can give you some interesting results to better understand what it is that they do.
-Tim