This question comes up every now and then, so I thought a post with for a workaround might be helpful. When exporting a metahuman from Unreal to USD you will likely encounter issues with the textures/materials on the face. This has to do with unsupported material nodes. We attempt to track these changes and include them in MDL/Kit to make the process as smooth as possible, but that sometimes can be a lengthy process.
In the case of the latest Metahumans, there is a material function called MF_AnimatedMaps. Within that function were MF_Headmask functions that included nodes that are incompatible with our current MDL distilling. In this video we link up the old function which resolves the export issue. Hope you find this helpful!