We lost a lower D1 shroud once when hard on the wind, in about 15 knots and flat water. The rigger later speculated that the ball end had siezed in the fitting, so one every loading the shroud would try to 'twist' and since the ball end couldn't move with it the rod broke just outside the end fitting. A quick tack saved the rig.
The boat was relatively new to us and we were uneducated on rod. Of course we immediately loosened the rig, opened all the ends to ensure they were free and coated with lanacote before reassembly. The rig was 12 years old at the time.
The other incident was actually a deck stay (from deck to mast step) inside the boat. Same problem in a way but this time whoever made the deck stay welded a fitting for the mast step tang onto the rod.. essentially created the same problem.
Rod can be coiled, and in fact if you stretch a section out on the ground it will hold a bit of a spiral until you attach/tighten it up. As a result there's a torsional 'motion' when the rod gets a load, esp a shock load like falling off a wave. The rod must be able to move inside the fitting otherwise it work-hardens and can fail. When we removed the failed shroud you could see where it had been cracked for some time before it finally let go, just inside the end fitting where it was out of sight.
But I don't think this should be alarming or indicates that there's an essential problem with rod for inshore sailing. It was clearly self-inflicted in a way, and as you indicated failures are quite rare. This just emphasizes the point that your entire boat (above and below decks) requires attention and maintenance.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"