I am having (apparently) the same problem. We are under Moodle 3.6. As a teacher, I can see the grades in the quiz activity when I look at them under Results of the quiz activity. The quiz activity appears in the Gradebook. But if I select grader view, single view or download (for example) from the Gradebook, all the grades are zero.
I tried moving the quiz activity to a different category in the Gradebook but this did not solve the problem. I also tried hiding and unhiding the quiz activity. Note that the quiz activity had some essay questions that required grading and we use the Grade Me plug-in.