I'm not entirely sure why this was happening, but it's likely a bug that will be fixed in the future. Since then, however, I've noticed that other UWP games sometimes default to the integrated Intel GPU, both on my Surface Book and other laptops. Thankfully, there's an easy way to fix it.

Here's how to force your Surface Book (or any laptop with an NVIDIA GPU ⁠— check out our best graphics card picks) to use its discrete graphics processing, and check whether your games are using it as appropriate.