Why do blind rivets break the mandrel and not at the breakpoint of the pull stud? I

1. The pulling force of the mandrel itself is not stable, the breaking point force is very close to the pulling force of the mandrel itself, or the heat treatment is not done, and the mandrel is brittle.

2. The mandrel has been damaged before the riveting.

https://www.yukerivet.com/news/the-mandrel-of-the-rivet-has-been-completely-pulled-out-is-it-qualified-to-leave-a-penetrating-hole-in-the-rivet-body-after-riveting/