Very common and more so than some other engines. The material (rubber?) between the outer ring and the hub dries out.
I had one outer ring fly off on the freeway and another just drop to the ground when I started the engine.
On this engine the belts give the outer ring a good tug everytime the engine is started. On an engine like a SBC, the belt pullies are in front and bolted to the hub of the HB.
Even before I added AC, I mounted an unused pulley to the front to trap the ring from coming off and going through the radiator.
