A UDN user has reported that a difference in appearance between two identical materials with the only difference that one uses material attributes.
I have boiled down the issue to FMaterialResource::IsTangentSpaceNormal having a different default behaviour when material attributes is turned on/off.
This is bizarre and needs investigation and the behaviour needs to be made identical again.
UDN: [Link Removed]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-190498 in the post.