I'm on Windows 10 and use the experimental version with DirectX 12 from Steam, if relevant, and find it very difficult to tab in and out of the game. Tabbing out will make the screen go black for ~1 second and is able to be reliably exited, however, tabbing back in will most commonly fail repeatedly.
I can click on the task bar icon, Alt + Tab, or any other combination of ways to bring the game to the forefront again, but they all result in the same thing. The screen goes black for ~1 second and does nothing, or after 3-5 attempts, it will finally bring the game back again.
My graphics presets in-game are Fullscreen, 4096 x 2160, 120 FPS.
I can provide more information if needed. I have a feeling this may be related to Windows HDR, as other games with similar tabbing in/out issues often stem from this. Its acting like its trying to tab back into the game, but something is snagging in the process.