Having the nut unscrew is not a common problem, but does happen. Usually related to outer wheel bearing failure. If the outboard bearing fails and allows the shaft to move in far enough so the nut contacts the dust cap, this will shear the cotter pin and allow the nut to unscrew.
If you decide to rebuild the final drive instead of replacing it, pay close attention to the case where the axles seal fits. Running for a while with a failed outer bearing will allow the axle shaft to destroy this area of the case. Also inspect the spur gears for chipped teeth if run with a fully failed bearing.
If you go with a used final drive, try to find one with the much stronger "smooth back" axle, as shown in the link I provided in my other post.