Duplicate the original face mesh that came with your MH and then open that new duplicate mesh, at the top of the screen you will see a reimport mesh button, dial down the menus for that and select reimport mesh from new file, now select your mesh fbx from where you exported from blender, you will get a texture dialog screen where you need to match up the texture names. Once imported reassign the control rig to the face mesh and all should be good.
If your mesh is slightly off I would suspect that it wasn’t rotated exactly 90 degrees or may have been previously moved without knowing. Let me know if that works.