+1 for contacting Warwick.
The reason seems simple to an engineer like me: overtightened nut.
The Zinc alloy that is used for the body and sometimes on the shaft, won't stand much pulling force.
Even less shear forces.
A fingertip (very precise I know
) torque on a short wrench is all that's needed to keep the nut and the tuner in place.
One can easily rip the inner thread portion off from the body with even a moderate torque, especially if the material is thick, the fit is snug and the HS is thin.
If the barrel is thin and/or the fit is not snug, the barrel either splits or the threads shear.
I had the splined portion of the tuner in my EpiBird tuner snap off once, but on Yours the spline probably gave away after the body snapped in half.
For gits and shiggles, tap the cover and the worm wheel off with a punch to see just exactly how much grease was "wasted" on that particular machine