I don't know if this is by design or just a special case, but when applying the following search filters score:0 is:answer closed:no the closed:no causes the results to show only questions, despite the is:answer.

Like I said, I'm not sure if this is a bug or if I'm misinterpreting some of the filters.

This is , as only questions can get closed, not answers. If you hover your mouse over the closed:yes options under Advanced Search Tips the tooltip says: