I've recently noticed a bug while selecting the option "Unanswered" in question's category (mobile device). Whenever I seelct it, the questions gets sorted on the basis of "Votes".
EDIT: It's no big issue as Martin has said, but when I move to next/prev pages, the questions aren't unanswered anymore.
But, selecting other categories works fine!
Android Oreo (Chrome), Android KitKat (Firefox). Jump directly to Edit #2.
Edit: (no more useful after edit #2)
The instance I clicked unanswered option (adjacent to All Questions heading) in select element,
The instance I clicked "pagination buttons" (adjacent to
15 per page text) to reload next/prev pages,
As you can see, it is showing some questions with green mark, which means they've been answered.