I've noticed the trap breaks due to exit of combat. If the target has other aggro options (your pet, a buddy, etc.) combat is not ended and only you go out of combat, the trap seems to stay active as all aggro is not lost. The trap should only break if ALL aggro is lost on all active party targets (combat terminated). This might explain why it still works in instances as well since you are likely in a group and the target would aggro onto the group.
This process is also part of a 'safe' pull. If the mob can't see the group to aggro onto them, you can FD and all mobs (in case of wrong or over pull) run back to starting position, freeze breaks if you used it. At least, that has been my observation.