Essay question and manual grading

I would like to report what I think is a bug but I could be mistaken.

If I go to manually grade an essay question and I think the student should get 0 for it. I leave a comment and click on the save button.

If I then go back to the question list, the number of ungraded questions has not changed.

If I however give the response a grade of 1 all is good and the number of ungraded questions reduces by one.

I would have thought that a grade of 0 would be accepted and this would mark the question as graded.