[Lynn Eades]

It is likely the inner tab was broken on assembly.The finish/ manufacturing style of the nut is a major factor
Some times the washer will turn with the nut rather than staying still with the inner race. This is enough to sheer the tab.
I would carefully lube the nut face of the washer, and leave the beaing face dry. The other thing to do is mark the washer where you can see it, to see that it hasn't moved after tightening.
A failing bearing could possibly turn the washer, tearing the tab off (if the inner race turns on the stub axle)The inner race, the lock washer, and the lock nuts, all turn as one.
I have seen similar set ups on other vehicles, where over time the preload of the bearings relax.the nut relaxes on the thread,(The bearing inner race wears the side of the lock washer away) and lets the lock washer become loose. The washer then flogs out the tab.
I hope this helps.