I had the exact same problem with my new Surface Book 3 with Nvidia graphics. Worked great for a few weeks and then it completely disappeared. The Surface Diagnostic Toolkit was no help at all - said everything was fine. Tried rebooting. Tried undocking the display. Nothing worked.
Then installed the latest Surface Book 3 update (SurfaceBook3_Win10_18362_20.051.9924.0) that was available to download. Rebooted. Everything was fine again.