This is a bug found by a licensee
1) Create a Material
2) Create a Material Function
3) Add the Material Function to the Material and connect its output to the Material base color
4) Open the Material Function
5) Add a Function Input
6) Change the type to Texture 2D
7) Connect the output of the Function Input to a TextureSample node
8) Connect the output of the TextureSample node to the Material Function output
Textures assigned to the material function input should now be fed through and outputted correctly. To reproduce the bug:
9) Add a re-route node between the Function Input node and the TextureSample node. This will make the TextureSample node output black.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44670 in the post.