High performance airbags often have the ability to inflate at more than one speed to account for the vehicle's speed and whether the occupant is wearing their seatbelt. If you're going slow, it inflates slower, while if you're going fast, there's less time, so it inflates faster. There's also a sensor to adjust this if you don't have your seatbelt on. Sounds like the thing is only using the fast inflate mode, which can act like a major punch, especially if you don't have your seatbelt on.
If I had to use that passenger seat, I'd move the seat back as far as possible and ensure the person was wearing a seatbelt to limit potential harm until the thing was fixed.