This is how GitHub has always worked, and it's been documented the entire time.
Forks are not just copies of other repos which go on to get their own history (or lack thereof) from that point on. They are linked to the parent repo, and share its visibility. If the parent repo goes private, so does the fork, for example.
There's a parent-child relationship that GitHub creates when you fork a repo using the "Fork" button in the UI. That is how this relationship is created.
This has always been the case with the "Fork" button on github.com. This should not have surprised you.
Forks are not just copies of other repos which go on to get their own history (or lack thereof) from that point on. They are linked to the parent repo, and share its visibility. If the parent repo goes private, so does the fork, for example.
There's a parent-child relationship that GitHub creates when you fork a repo using the "Fork" button in the UI. That is how this relationship is created.
This has always been the case with the "Fork" button on github.com. This should not have surprised you.