Steps to reproduce:

1. Open the attached project
2. Build and run the scene with these settings (should be set by default):
D3D9 Fullscreen Mode: Exclusive Mode
Graphics API: Direct3D9
Run In Background: Enabled
3. Switch to desktop (Alt+Tab)
4. Open the 'output_log.txt' for the player ([BuildFolderName]/[BuildName]_Data/output_log.txt)
5. Notice once the player was left in background it stopped executing Update() functions and "HandleD3DDeviceLost HandleD3DDeviceLost: still lost Skipped frame because GfxDevice is in invalid state (device lost)" was spammed every frame instead
6. If you switch back to the game window and switch to the editor again (leaving game window in the background), you'll notice that it continued executing Update() once it was the active program again, and stopped as you left it in background

Notes: Does not happen if Fullscreen Mode is set to Fullscreen Window. Also does not seem to happen under D3D11 graphics API.