@oslera
It sounds like you did everything correctly. for the assignment select the post grades method of manually. If any grades are visible, click on hide grades and then you should be good to go until you select post grades. That is exactly what I do, and I have no problems with it.
Just to cover all the bases, did you go back and check to make sure that the grade posting policy was actually still set on manual? Possibly it wasn't saved or it was automatically converted back somehow. Not saying you did anything wrong, but what you said you did is exactly correct and you should not be having an issue.
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