It’s not some side benefit, it’s the primary reason for private browsing to exist at all, at least as far a product manager (who actually understands and cares about what users actually want from their browser – so not actually a bad one) might be concerned.
That’s why I can absolutely understand how this kind of bug can creep in. The most important aspect of this feature (from the point of view of most people actually using the feature) is not that browsing leave absolutely no trace anywhere, the most important aspect is that browsing leave absolutely no trace where other users of the PC can see it†.
I’m consequently a bit confused why everyone is unpacking the big conspiracy guns. This seems like an easily understandable and completely plausible bug to me.
People often don’t use separate OS accounts (it’s often just not necessary) and even if they do they will often still share the PC from time to time (even if just for a brief moment) or just show something off for some other people.
During all those occasions the browser history could be visible, even if people aren’t really prying (autocomplete, frequently visited websites, …).