Material preview doesn't render correctly with using a Static Switch Parameter with Texture Objects. See attached image
1. Create a simple Material Function:
Texture 2D Input->Texture Sample->Function Output
2. Use this function in a material a demonstrated in the attached image
Result: Plugging in the TexObjParam directly works fine, but feeding them into a switch first causes the Material Preview to be black
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-20532 in the post.