when you change positions you have a different control mode setup running. three ways to fix it,
go to options/controllers/map controller 1 click on the center check box and turn "auto mode select" off and it will maintain mode 1 or...
on the same page use the drop down "mode" box to select mode 2 (vehicles/gunners) then click the red button to the left that says "copy mode one" or....
set all of your controller buttons and switches for mode 2 manually so that you have a unique setup while in vehicles and gunner positions.
Good luck