the bottom of those two walls, I see this all of the time.
It looks like your directional light is going through the bottom of your terrain :0 Same thing happened to me and it was confusing until I figured it out but I haven’t found a suitable way to fix it other than not have it pointing upwards at all.
Thanks a lot for info! That seemed to be the problem!
I had the same issue. I placed a large box under the ground level of my terrain and it solved the problem. Rude solution though.