When MaterialInterface is added to UserParameter of NiagaraSystem, Material can be set in Properties.
Also, in UE4main CL15988344, there is no MaterialInterface item in the NiagaraParameterCollection to begin with.
1. Create a NiagaraParameterCollection.
2. Add MaterialInterface.
3. The item to set Material does not appear.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-113675 in the post.