I have the same exact problem.
I need to export a combined skeletal mesh out of UE and the material of the head is broken - instead of using the material/textures of the head it puts the body material on the head.
I dont know what do do anymore, I think I have tested all possible combinations.
unchecked Hair LOD in project settings
full rig or joints only
2k, 4k, 8k textures
assemble not assemble before the export
but the problem is there even if I create a completely new project, settings and new empty default metahuman!
it happens both in 5.6.1 and in 5.7 preview
what am I missing???
thanks a lot!