I use the "New Quizzes" interface both for routine "reading questionnaires" and for (timed) online exams. In the past week, I've had multiple students encounter a bug where they do a quiz and submit it, but it seems that the submission is somehow not transmitted back from the New Quizzes engine to Canvas itself. The student gets no warning that there's a problem, and the rest of the class has no issues.
The result is that the Canvas gradebook shows the assignment as "missing" with a zero grade, and the Canvas SpeedGrader interface also says "no submission". As an instructor, I have no reason to think anything is wrong: it's exactly as if the student skipped the assignment. But if I open the quiz itself and go to the Moderate tab, the student's work is right there as "Attempt 1/1" just like everyone else's. And if I click through to view their attempt, I can assign a grade there (either by grading a question that requires manual grading, or I think by adding a "fudge point" and then taking it back) and click "Update": at that point, the score is transmitted back to Canvas and everything seems normal (except that the assignment may be marked as "late" by Canvas, so if I had a built-in penalty for late work I suspect it would be triggered).
Now I'm worried that this could have happened to some students last fall without me even noticing. And I think I need to go through every single quiz I've given this term to check on every single zero: it's a tedious process. Just to be explicit about this, it has hit three of my students on at least two different assignments in the past week, and I haven't even started looking back at earlier work yet.
I think that this issue is also being discussed in comments on this other post. I'm submitting it as a new issue here because I think the original post in that thread was an entirely different issue (a missing due date preventing any scores from showing up in gradebook). Someone in that thread says that Canvas phone support told them it was a known bug, but rare. My experience this week makes me nervous about the "rare" part.
In my experience, Instructure tries to fix Known Issues as soon as possible. For now, I'm glad you know the "apply 0 Fudge Points at Moderate" workaround.