Units that have enabled dynamic simulation (so they do stuff when player is near and freeze when he's not) are frozen whenever player is a passenger of AI-controlled vehicle, or when both player and AI are passengers. Dynamically simulated units will stay frozen as long as player is in that vehicle, leaving it in any time enables simulation again.
Dynamically simulated vehicles with dynamically simulated crewmen also won't move.
Very important thing is that this does not occur with base Arma 3. Once any DLC, Creator DLC or mod of any type is loaded, this bug will happen.