As soon as you hit page 8 (of 50 results per page) of unanswered questions and beyond, all the questions have answers; is this a bug?
That list shows all "questions that have no upvoted answers" (as the tooltip states). So this would seem to be correct. Whilst questions from page 8 onwards do indeed have answers, none of them have been upvoted!
To get just the list of questions that literally have no answers at all then the tabbed "Unanswered" link at the top of the page returns this: