I ran into the same problem (I am still on UE 5.3). On a very fresh tiny project.
It happened on a skin material instance first - I do not know if from a pure coincidence or there’s something else to it, but even after creating a completely new project (just a template scene with imported few materials and single FBX character), the issue repeated itself on the very same material instance.
It either cached something outside the project or I was just unlucky (recompiling master material helped).