Initially I trusted the game to auto-pause as it had done previously but this does not work with an interface like a chest open.
What I excepted:
When alt+tabing out of Minecraft with a chest open the game should have paused.
What happened:
The game keeps going.
To reproduce:
1. Open the inventory.
2. Alt-tab out.
3. The game remains un-paused.
This has been around since Minecraft was created, it's intended as a feature not a bug. Back in the day this is how I kept iron cooking while I browsed the internet.