Hello Charlie,
I checked the progress of this issue this morning. The problem is that the various APlayerController::GetHitResultAtScreenPosition() functions do not set bReturnPhysicalMaterial=true on the FCollisionQueryParams so it is never looked up.
This issue is marked as backlogged. I do not have a time frame for when this will addressed. Currently there are many issues which our developers have deemed a priority and they are handling all issues reported as fast as they can.