One of my students had this issue during the week, she received 5 points out of 5 but the test didn't grade her accordingly.
Last week I had a similar issue, but the grade on the grade book was zero.
I have grades on the same activity from two weeks ago, but new submissions are getting a zero.
I already checked some of the settings for the quiz. I can tell you that the preview mode is deactivated, it's supposed to keep the highest grade, the students have 5 attempts and no due date.
To "solve" it, I eliminated the quiz and copy back the grades, I'm wondering if there's a permanent solution.