It is hard to tell from the pic if the valve stem tip is concave or convex.

In any event, the wear looks like it is entering the top groove of the valve lock so there is a real risk the locks will fail.

What is the "pre-existing factory defects with the cams and buckets" ?

As to what caused the failure, I vote for that lash adjuster to have collapsed, engine run with a tapping valve and the fatigue broke the bucket through / hammered the valve tip. ( Remember, the lobe contacts the outer diameter of the bucket first leaving the center unsupported. )