Have ran into cracks in the ring similar to yours.
Have also ran into cracks in bulkhead but the ring was ok.
Have found the odd rivet head missing, ring & bulkhead ok.
Only once I found crack through both but that spinner definitely showed signs of being over torqued.
I know the cause of the last one, would be curious what caused the other 3.
built a few RVs, rebuilt a few more, hot rodded more, & maintained/updated a big bunch more