Hovercraft "jump" when passenger seat ist used?

Hi folks,

bought the game together with some friends and we're having a good time. One question:
I can't use my Hovercraft as long as a passenger seat is in use. It starts to behave weird and at some point just flys straight upward, I've even reached 10 to 15 meters. If the person leaves the seat, everything returns back to normal and it's perferfectly manouverable again.

That's a bug or are we doing smth. wrong?

cheers