I have an issue with the rudder servo on a plane I just purchased that has me scratching my head, need some help.

If I take the rudder stick and deflect it right or left a little, then let go of stick, the rudder returns to center as expected. If I take the stick and fully deflect the stick to left, and then let go of stick, the rudder returns to a point that is about 20 degrees to the right center. Then, if deflect the stick ful right and let go, the stick returns to dead center. Power is coming from a fully charged LiFe battery.

All other servos act normally.
Tried 3 different rudder servos this afternoon, same result
Replaced servo cable between receiver and tail of plane where servo connects to it, same result.