When you loading into a world, and then unfocusing the game window (by opening start menu, clicking the taskbar, open another apps, etc), when the world is fully loaded the game didn't pause the game, instead it will continue to running in the background.

Steps to reproduce:

1. Open Minecraft Preview
2. Enter into any worlds
3. When the world is loading, unfocus the game window by clicking the taskbar
4. Wait until the game fully loaded the world

Expected results:
The game automatically open the pause screen and paused the game.

Observed results:
The game continue to running in the background.