Surely the issue is that if the bearing seizes then your tail rotor pitch control is now jammed. Whereas if it is just the nut coming off then it means that control is lost.
The bearing is seized in the rotational sense as it isolates the static and rotational part of the control. Then the rotational torque is transferred to the static part of the shaft and could unwind the nut as the torque is now applied
to the static shaft.
The nut connects the static pitch change to the feedback lever. The feedback lever cancels the pilot valve input once the piston reaches the desired position. Standard servo actuator behaviour.
Without feedback the pilot valve input will not cancel.