Right now when creating postprocess material one can set BlendablePriority in the Details panel. However if material instance is created using the postprocess material, then this option is not available.
Consider following setup. I have postprocess material with the bunch of params (scalars, vectors, textures…). Then I create two material instances from it. If I use either of those two instances separately in the PostProcessVolume then everything is OK, however if I use them both, something is really wrong. Look fro the images below to see an example.
1st material instance alone:
2nd material instance alone:
Both instances together - bummer 
