I suspect it depends on your region, page/post in question and browser fingerprint. A post marked as public isn't 100% guaranteed to be publicly viewable. Sometimes you can view it but merely scrolling down on the page would trigger a login form for example (I've had this happen for pages that are definitely meant to be public such as businesses who'd have an interest in getting as many eyeballs as possible on their content).

I might be wrong and maybe the behavior is actually fully deterministic and isn't nefarious, but knowing the company behind it I'll assume malice until proven otherwise.