This is a weird issue that i'm not quite sure how this mod causes. I'm using RJW (not explaining that one if you don't know) But it adds new body parts. The parts are added to every pawn as they should be even alien ones until i add this particular mod. In fact somehow this mod stops any hediff from being generated on new pawns such as addictions, scars or health conditions. I have done testing slowly adding mods to find the culprit and I was surprised to find it was this mod.

If needed I can add the mod back and give you the log if one is generated on load.

This mod does not touch any of those things, even remotely. BTW I have played with RJW and not had this issue.
I am not touching anything that even touches pawns. This is purely affecting one part of one def regarding materials.
So you have to be mistaken on which mod is causing it. :/ I really don't know any other answer there.