Niagara will not work with Volumetric Materials.
Found in 4.20 Preview 5 CL# 4173640 and 4.21 CL# 4207835
The Niagara emitter will not show the volumetric material applied to it
The Niagara emitter would be visable using the volumetric material
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-61771 in the post.