Earlier quoted context omitted.
"This unfortunately makes sense because it is a private repo." I disagree. I expect a (i.e. my) fork to be independent of the original repository, no matter if it is private or not. It's enough if a fork of a private repository is private then too.
Don't use the "Fork" button in the GitHub UI, then. It is intended for collaboration and establishes and maintains the parent-child relationship of "your fork" and if the parent repo is deleted, so are all forks. If the parent repo is private and goes public, so do all forks. If the parent repo is public and switches to private, so do all forks. This behavior is laid out in docs.github.com and is not secret. This has…
Regarding forking a private repository with a public repository, it's a corner case for sure. In my opinion it's best to forbid forks of private repositories at all, and forbid to make a repository that has forks private, than to create problems like the one of the user in this topic.