When using a Two-Sided Material and Shadowing Two-Sided for a Plane or Landscape, with non-flat normals, there are parts of the material which are still illuminated.
I have provided a test project so you can simply rotate the light source, rebuild the lighting, and see this issue in effect.
1. Download and open TwoSidedTest.zip
2. Open project and rotate the light source so it is coming from below the actors placed in the world.
Expected The top of the plane/landscape is completely dark
Outcome Parts of the surface are illuminated and visible
Head over to the existing AnswerHub thread and let us know what's up.