Atmospheric Fog actor creates a black ring on the horizon

When using the Atmospheric Fog actor, a black ring can be seen on the horizon. This can be seen by simply placing the Atmospheric Fog into an empty level.

The issue cannot be seen if there is a skybox in the level.

This is occurring in UE4 Version: 4.12.0-2970308+++UE4+Release-4.12
as well as Unreal Tournament Editor Version: 4.12.0-2973693+++depot+UE4-UT-Releases

STEPS:
1) Launch Release build 2973693
2) Load CTF-Facing Worlds
3) Observe atmosphere around earth

RESULTS:
Black ring appears around planet