I’ve been trying to replicate your issue with a simple asset I’ve made but cannot get the same results.
I am currently testing with 4.5.1.
I created a sphere with a two bones and assigned two materials similar to your character head. I’ve setup both materials to use tessellation. I tested with PN and FLAT with no issue.
Are you able to reproduce this on a simple asset or anything other than your skeletal mesh? I would like to narrow down if this is a issue with the skeletal mesh or possibly a bug.
Thank you!
Tim