I just experienced this today 10/29/20. I'm new to Canvas since the pandemic began, so I'm still learning all the intricacies. When I made it to this thread, I was a bit surprised to find out this has been an issue for more than a year and it doesn't seem to be getting fixed.
It sounds like this is caused perhaps, when you enter grades manually, even though Canvas grades the quizzes for you. In my case I had two students who completed the test or almost completed the test just barely by the deadline, and may have been shut out before they hit the submit button. I don't have a penalty applied for late submission. The history shows the tests were graded accurately at first, but both scores changed to zeros at the same time the next day by being awarded negative fudge points equal to their scores. I do remember manually awarding zeros the following day to the students who had not taken the test. But since they had taken the test and they were already scored, I wouldn't have touched their scores. I don't remember if the time-stamp the change occurred is also the time I was entering zeros for the missing tests.
Someone should fix this glitch. We shouldn't have to worry about this happening everytime we give a quiz.
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