Hey, I recently made a day/night cycle through a directional light and I experience this problem at night. Screenshot - a5a60aa75a50ced7ac22cd18884d34c3 - Gyazo
Screenshot - 987477c14d975ec049b97517ba9c91bf - Gyazo
I already have the landscape casting shadows as if it were a two sided material. Not sure what is causing this, any ideas? 
Hi alanconrad,
I have had this issue too. I fixed it by placing a box BSP under the terrain and making it the same size. This will block the light instead of the terrain.
Hope this helps