mmm... I do ocassionally use an external monitor, but I have this problem regardless of being connected or not to the monitor. Luckily, in my case I do not need to force a shutdown, I can attach/detach the tablet and then the surface can wake up if I press the power button right after that. It's faster than a full shutdown... but not ideal.
Not sure where it's coming from... but my guess is that this is related to the multiple wake up/sleep problems reported in surface devices, and not to the video card. But honestly I have no idea...
Best,