interesting?!
Was originally going to mention sometimes with a pos disp blower if you get a backfire it can lock the blower sudenly and strip the key outta the pulley. This happened on a mates 350 chev with a 6/71 with big toothed belt, where as a GZE belt would probably slip before it would strip or break a crank. But it can't be that if it happens without s/c as well. I'd be thinking some sort of harmonics + maybe poor design and or materials/manufacturing standard? (strange for toyota!?) Maybe even bad tolerancing on the keyway fit?

Taken from mazda miata website:

"Typically, the key wears the keyway slot in the crankshaft nose. Worn keyways are not repairable. In other cases, the pulley bolt will fail to stay tightly screwed to the front of the crankshaft. Insufficient tightening torque upon assembly is an obvious cause. Another less intuitive failure is excess torque can stretch the bolt which causes looseness. In some of the incidents, inserting the key into the keyway upside-down resulted in an interference fit (Figure 2). This improper assembly can stress the bolt, causing the head to break off. The misalignment also causes destructive imbalance. The imbalance can break the crankshaft. I should point out that some key and slot combinations are such that the key will fit in the slot either way without interference. It is also possible that v-belt tension and the forces necessary to drive the accessories may cause cyclical loading that leads to a fatigue failure of the crankshaft nose itself. We have closely examined perhaps a half dozen crankshafts. It is often not possible to determine what part of the assembly failed first."