If a focus transition in full-screen mode triggers a lost D3D9 device, the HUD and MFDs disappear. The re-appear as soon as the simulation leaves paused mode.
Most likely,
any full-screen focus transition causes D3D9 to lose the device context;
lost devices lose all non-managed textures (like render targets);
therefore, all virtual screen textures are gone until the simulation renders them again.
Before Alt+Tab:
2022-06-05 foreground issue 1.png (131.2 KiB) Viewed 8571 times
After Alt+Tab:
2022-06-05 foreground issue 2.png (119.63 KiB) Viewed 8571 times