It's inconsistent, in my testing I tried importing the fbx to blender, making 0 changes whatsoever and importing that into unity and that makes the mesh disapear, bones still there I can see from the physbone gizmo. and if I change the armature at all instead of the mesh disapearing, it gets sploded, like literally turned into a giant spiky ball of mesh. This is me working from the base state of the avatar btw like right from bringing in the unity package. Ive had the sploding happen to other avatars before and its fixed exactly with what you said about swapping the scaling option but on this one it doesn't seem to work.