So yeah, I'm not surprised by those results.
Not super exciting, unless you need ammunition when arguing against some kind of non-benevolent project dictator.
The majority of PRs tend to not affect the build status.
The rare cases (4-5%) where it does, is a complex architectural reason, or the tests are variable. Which doesn't even factor into this research... Such as tests that rely on timestamps/external requests/etc that randomly fail, yet are still a part of the test suite (pretty common).
The places where people are afraid to contribute generally tend to have overly complex contribution guidelines, rules, and acceptance criterion where you must sign a waiver, agree to their code of conduct, submit some form of a RFC... rather than the CI.