When I move my critter's head, some vertices at the back of said head move along, creating a fold, despite having no weight assigned to the head bone (DEF-spine.011).
Rest pose and lack of weight shown here: https://imgur.com/a/He6T31J
Rotated head bone shown here: https://imgur.com/a/F8omPKH
The head bone is the only moving bone here and all the weight from vertices of that area is assigned to the first neck bone, which is stationary, so the area should stay flat. I used auto-normalize when weight painting and double checked all vertex groups to make sure there is no weight from some random bone.
I have no idea what's causing this and how to fix it, although I'm still new to blender so despite my effort to find out, it might be something obvious. Weight painting still "works", adding weight to the head bone in this situation will still move those vertices, but I can't seem to eliminate that fold at the back of the head when it's tilted back, regardless of what I do (it's not present in the rest pose).
In case it matters, the rig was generated by Rigify.
I checked other similar questions about mysterious weightless deformations and saw they usually still had some other sneaky bone with assigned weight or the observed bone's weight wasn't quite 0, but from what I can tell, that's not the case here.