I have been playing with it & trying to figure out this issue through trial-and-error. And, I think I've got it (based on some feedback above).
Apparently, when I put in zeros directly into the gradebook, Canvas views that as a manual override (at least this is true with the little experiments I've been doing this with my different class sections).
If we teachers want to insert "temporary" zeros that are still malleable to the Canvas quiz settings of allowing students multiple attempts for their highest scores, the zeros can't be put in directly into the gradebook, but rather must be put in through "speedgrader". MOST of the time it has updated my students scores from this speedgrader-entered-zero when they eventually took this quiz. A couple of times I had to (for some reason) manually update using the red "Update Scores" button on the bottom of the quiz in speedgrader. (But I could at least SEE in the gradebook the "submission" icon for that student so I knew to go into speedgrader to update).
I hope this makes sense & this might be something good to tuck aside in case someone else has similar issues.
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