Hey!
I experience the same issue with a directional Kuwahara shader!
To be honest, I was quite happy to find this topic and to see that this seems to be in issue with SM6 - i though my shader implementation sucks big time! 
Anything came up about this issue yet?
I switched back to SM5 but get warnings that Nanite and Virtal Shadow Maps won’t work unless I switch to SM6.
With SM6, my PPM performance drops and is unusable.