Thanks for raising this issue, this is an interesting and quite tricky problem.
I don’t see a reason, why this should not be accepted.
My first feeling is the same. And in fact, even if one can come up with something not behaving as expected when the Child class is nested into the Parent, differentiating the case where it is or not a problem would be tricky. I therefore created a ticket (SONARJAVA-3580) to not report an issue in such situation.
If someone is interested in this topic, feel free to dig further, if you can come up with a concrete example, I would be happy to follow up on this.