Sorry to hear everyone is still having this issue. I have the Surface Book 3 15" with NVIDIA GeForce GTX 1660 Ti with Max-Q Design.
I have to admit its been getting worst recently. I dont really detach the screen, however its the only general fix when the GPU disappears. Its as if the laptop just loses recognition of the base with GPU, although the keyboard is find. Re-freshing device manager does nothing as GPU not found.
I also find the general screen detach/attach is not that reliable , hence why I dont do this. Weird as my original Surface Book 1 with Nvidia GPU was rock solid no issues.
Think I may try using Nvidia native drivers, and perhaps check for new firmware. I actually spoke to Microsoft chat , what a waste of time. They actually wanted to replace the hardware, really ? No wonder Microsoft loses business, they need to get their hardware and support up to scratch. Its a shame as love the design , screen and keyboard. Will update if I have any success with drivers and firmware .
Looks like a new firmware update dated 14/6.