Hi there,
That would need a bit further diagnosis, but I would check to make sure that the joystick is not malfunctioning. It would take a little bit of ingenuity, but you should be able to switch the bow thruster plug (assuming the bow thruster works properly) to see if that solves the issue. If it does, the problem is with the joystick or wiring. If not, then you have an issue with the thruster. Hope that helps get you started.
Cheers!
Ralf