Could be the system is bugged or that something is in between the landscape and the light at the distance the camera is at to make the map render happen.
Or it could be the render distance - if using meshes and such, sometimes even landscapes if you customize them to load differently than default (they ignore Z hegiht by default).
If neither is the case, it could be an engine bug. Try and create a new level/system to see if it works with just a light and a landscape.