A common fault. I've seen it on most MD22 (B only, A has the pump directly on the camshaft).
I've epoxied ours back on with a reinforcement of metal epoxy and eventually spraypainted the repair.
I think they probably get too much rotational force on the cover when the big bolt on the other end is torqued down to fix the pump and tension the belt. Our repaired one has held for a few years now, but it's much thicker than before. Does always seem to pop up onto the engine rather than down into the belt, luckily.
