I was going to complain about the same thing! Thank you for asking. Just a shame no one has yet gotten around to providing an explanation or (better yet) find a solution to the problem! I imagine it can be concerning to students as well, when they have done the quiz to find Canvas telling them it is "x" (incomplete).
I have color coded my gradebook so at least I can see the difference between an "X" because it is "missing" vs. an "X" that was done (or, as I am seeing) done "late".
My one "new quiz" is also "graded" as "complete/incomplete" is set up as a survey but does include an open ended question that thus required manual "grading". I have not yet experimented with a graded "new quiz" that might have an open ended question (and I hesitate to use "new quizzes" for such purpose until the bugs are worked out of it).
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