I think there may be a graphics card driver issue with the Surface Studio 2 or possibly a software issue with the Windows Mixed Reality Portal and its ability to output video over a USB-C port.
I recently upgraded from a Surface Pro 5 to the Surface Studio 2. My Lenovo Explorer Windows Mixed Reality headset doesn't work with the Surface Studio. I've tested and confirmed that it still works on the Surface Pro 5 (using the official Microsoft mini display port adapter). Seeing as how the Studio 2 doesn't have that port, I'm using the Microsoft USB-C to HDMI adapter on the Studio 2. The headset it recognized. Tracking works just fine, but in the HMD itself no image is shown. The back-light turns on in the headset, so it's just showing a slightly illuminated black screen.
I'm running Windows 10 Pro 1809 with an NVIDIA GeForce GTX 1070 graphics card (driver details in the screenshot below). There doesn't appear to be any driver updates available for either the graphics card or the headset. I checked both by just looking for driver updates in device manager and manually at Lenovo's site. All devices listed in device manager are properly addressed (no yellow exclamation marks).
Looking at the reviews for the USB-C to HDMI adapter, it seems like other people are having problems with Mixed Reality on Surface products that output video over USB-C. One reviewer mentions having an HP 1440 MR HMD which doesn't work on a Surface Book 2. That makes me kind of suspect that the problem may be within the Mixed Reality Portal software itself (rather than the graphics card driver, port, or adapter cable).
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!