Which points to the fact that the first domino, is the pre-load end cap coming off. Then the splined interface and pinch bolts, seem to fail at their respective tasks, which causes the crank arm to walk off.
I'm also not a mechanical engineer, but given the forces and leverage involved I suspect the opposite is true. The walking of the crank arm under load is either deforming or unwinding the end-cap as a symptom, not a cause. Once the pinch bolts are tightened, you should be able to remove the pre-load cap and keep riding with no ill effect. The same is not true for the pinch bolts - if you removed those the end-cap would be destroyed in a few pedal strokes.
The Rise upper suspension pivot arm is another (not so good, as it also suffers from walking off) example of this design concept - the pre-load cap is temporary and is designed to be removed once the pinch bolts are tight.