Section of terrain goes black at night?

Here is an example.

Not sure exactly what is causing it. It seems to happen when the sun (directional light) is gone but there is also another directional light (the moon). It happens in the editor and the packaged game.

Any ideas?

In the image on the right, is there some sort of visible seam from just the left of the bottom right corner?
Assuming there is, could it be related?
Throw in a skylight and see what happens?
(just thinking out loud hereā€¦ good luck!)