To reproduce:

1. start minecraft and  an other program (eg. word).
2. enter a world.
3. While the game is loading, quickly open and close the other program befoore the world is loaded completely
4.  notice the world start paused

I would expect that the game only starts paused when the program is still open when the world is loaded. It doesn't makes sense that the game starts paused when no other program is opened.