Two and a half years ago I had this problem: Double decals: Have any of you ever seen decals render behind landscape? - Epic Games Forums
I was able to mitigate the issue a bit by setting the decals’ depth bias closer to zero, but zooming out further kept bringing the problem back. That becomes a major problem for me when I go to my world map level and zoom out over 10,000 UUs from the player’s army.
The issue ended up being depth precision. One solution is to increase your near clipping plane distance. If you increase your near clipping distance in Engine.ini, then the engine will shift its precise-depth area out.
Set the near clipping distance in Engine.ini to whatever makes sense for your game. Changing the near clipping distance dynamically requires native access.