Post process volume priority doesn't affect post process materials.
Found in 4.19 CL# 4033788, 4.20 CL# 4302132, and 4.21 CL# 4369303
The post process material is rendered upon application to the second volume
The post process material would not render because it is set to lower priority
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-64132 in the post.