Character has materials when I import, but at some point during animating and stuff the materials are replaced by this grey-checkered material. Changing them back here in viewports does nothing. Why is this happening?
很大程度上是模型本身的问题，建议你把模型导入到blender里，然后把材质修改成blender的，在导出fbx模型
Could it be that the shaders are updating?