Well that rules out user error, have to check

I would look at the restrictor plates, are they both identical and aligned the same way? Maybe try swapping them and see if it moves the problem to the other controller. That would isolate the issue or rule out the plates.
Is the joystick hitting anything? CP top, plexi, etc?
You could do the same thing with the wire harnesses? Swap them and see if they are the issue?
I'm just throwing out ideas that might help locate the issue.