BPs in sub levels freezes

I have added sub-levels to my game and it works fine, except in one way:
If the players are loaded into a level streaming volume that loads in a level, all the BP actors in the level functions, including if you walk out of the volume and then in again.

However, if you start the game outside of the volume, then walk into it, the BPs are frozen in place, without any functions running.

Why is this?