Why would this indicate a defect of some sort?

It's part of the stock design of most spinners.

The cap screw pulls the spinner down against the back plate.

If you tighten the spinner down too much ( which I did because of it loosening all the time ) then the outer part of the back plate is pushed backward while the central part is held fast by the prop screw.

That bends the back plate "back" to the point that you'll get stress fractures radiating out from the center.