Windows MR on Surface Studio 2

I'm happy to report that the problem has been resolved!

It was indeed a driver issue. Checking for driver updates for the Surface Studio 2's GeForce GTX 1070 card in Device Manager indicated that none were available and my own manual searching of NVIDIA's site didn't find anything initially.

I installed NVIDIA's GeForce Experience app and let it find the appropriate driver for my card. Sure enough, a new driver was release on January 11th! (Screenshot attached).

Updating the driver, however, didn't immediately resolve the issue. Opening the Mixed Reality Portal displayed the Windows logo in my HMD (but only briefly) and then it went back to the solid (but backlit) black. I tried rebooting, but that also didn't help.

Finally, I uninstalled and then reinstalled the Mixed Reality Portal. Now everything's perfect!