We have seen the same issue. One thing that we have noticed is that if you even click or tab into that field then it will override the grade. I see there is a fix in this forum, but when I clicked on it the page was not there. It would be VERY nice if the calculated fields could not be overriden unless you click on the editing pencil to access the that screen. It is too easy to override grades accidently and then the calculation no longer works.