Mine does the same. If you get the car moving just a little and tap the brake, I think you will find that the seat belt will be locked and you won't be able to pull it out while holding the brake pedal down. I don't know it for a fact, but I believe the seat belt relies on inertia (maybe a pendulum) to lock it.
Try that and let us know what happens. If you still don't trust it get another seatbelt.