We had the same problem the other day (using Moodle 3.7.6). The quiz was graded and the grades were available in the quiz. But the quiz grades in the gradebook all showed 0. The issue was with the multiplicator in the quiz grade item. It was set to 0. When
this was reset to 1, the correct quiz grades now appeared in the gradebook. It looks like this is the same issue here -- in the screenshot posted earlier, the multiplicator is 0.
The screenshot below also shows this:
