@JLegler -
I did some more playing around with this issue and have some more information. Evidently the assignment of 0 only works if a grade has never been entered for a student (and then taken away). I created an assignment only for my dummy student. made the due date earlier (I have not checked for a due date in the future to see if the 0 is reported right away or after the due date). In any case, the 0 is assigned to the student and I see the grade as 0 and the student does as well. If I change the grade on the assignment and then delete it, the 0 is not reassigned. So that is probably why you are not seeing that affect on current assignments that you have manipulated grade wise.
In writing this, I decided to check an assignment with a due date in the future. In this case I do not think the 0 is applied.
In all of this I will say that that feature is not something I would rely on to be correct. Also, it may only apply for assignments that require a submission - not just assignments in general.
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