I have seen it 3 times, in addition to what has been reported on this forum over the years.
All were early C5 models.
All dropped the exhaust valve head.
Two of them we repaired the heavy damage to the heads, and the customers rebuilt the bottom.
One of them is a warranty engine that I got during a clearance sale. It suffered this kind of damage during warranty, and they replaced it. I use it as a parts engine, and for reference.
The two we fixed had been ridden on the highway for all day in hot weather. I don't know the circumstances that killed the warranty engine I got.
Seeing you are from Australia, you should know that Australia is known for killing Bullets. Apparently there is a lot of hot weather and people get on the highways and drive at fairly high speed for long periods. The old iron barrel Bullets couldn't take this, and dropped like flies. The UCE does a little better.