Hi there,
The tk2d lit shaders are plain surface shaders, and it works the same way as any of the other lit shaders in Unity.
1 - You can try one of the unity default shaders to see if that works better.
2 - check that you have generate normals ticked in the sprite collection.
I'm guessing one of the above will fix it, as it works fine in my tests in Unity 5.10.