I don’t know exactly what to call this but I would not call it a bug or a problem, perhaps an oddity.
StackOverflow allows for tags to subsets of questions. In using just such a URL the OneBox showed something which doesn’t even have the correct tag and was not what was desired. I would suggest a change, but I have no idea if even changing it is worth the effort as there are other ways to present a user friendly URL.
As such just noting this here incase others find similar.
As URL in raw post. What was posted as raw text is one space then the URL which is converted to what follows.