Interaction between FSMP and head parts with non-standard skinning nodes

[Frosferes]

This is a quite deep-seated and confusing problem that I'm hoping someone might be able to offer some insight on. My mod Total Werewolf Overhaul adds actual head parts to werewolves as oppose to the heads being baked into the body meshes like they are in vanilla Skyrim. These head parts have several non-standard skinning nodes, by which I mean nodes that are not present in vanilla head parts. These non-standard nodes include a jaw bone, tongue bones and ear bones. These nodes are animated by the behaviour graph like any other non-HDT animation bone. There are currently 3 active head parts: The actual head, the eyes, and the mouth.

 

Now for some weirdness. If FSMP is not installed, or is otherwise non-functional, the following bug occurs. It appears as if a single ear node is not skinned and the vertices weighed to it stretch away to the origin:

 

 

However, if the FSMP install is valid, then this bug does NOT occur, despite SMP physics not being enabled on the nodes in question. I believe this is somehow related to FSMP having special interactions with head parts to facilitate non-standard skinning nodes like those which are present in SMP hair mods. The strange thing is this seemingly only affects a single node at a time, despite several other non-standard nodes being present in multiple head parts.

 

Monster Facial Animations by CinnaMewRoll circumvented this bug by removing the ear nodes from the head parts entirely, but I am looking to figure out the root cause of it as it has been a thorn in my side for a while now. If anyone has any leads on this, please let me know.

 

Another point is CTD related issues. I have been trying to enable RaceMenu functionality for werewolves as part of my mod. In vanilla Skyrim, attempting to open the RaceMenu whilst in werewolf form immediately CTDs. This I assume has something to do with the lack of valid head parts, as if MFA or my mod is enabled, then the RaceMenu will actually open and be configurable. However, there have been several times that these head parts have caused CTDs when opening RaceMenu, specifically and only if FSMP is active. The crash logs always points to some kind of culling issue with the 'head' head part:Crash_2022_12_16_20-2-40.txt

 

Would anyone have any clue about the source of these bugs? If not, is there someone else I could ask about this?