I had the same issue and it wasn’t related to DX12 SM6 pr Material or Shader compatibility, it seems the project was stuck displaying the fallback mesh only.
My Fix: Go into editor preference and change the location of the derived data cache for both local and shared, this will then ask you to restart, once it restarts it will force a rebuild of the shaders and meshes, this should fix the meshes being stuck in its fallback state.