Canvas Support, after looking at video of my professor grading, responded:
"Thank you for that. The main reason we will see some of the annotations be removed when the submissions are left and then come back to will be because the submission page was left before the annotation was completely saved. This can happen if the page is left too quickly, or if there may be trouble with the connection. To avoid this, they will want to make sure that the top of the submission page says "all annotations saved". We've done some testing as well and haven't been able to reproduce any trouble with annotations saving with any consistency. Let us know if you have any other questions."
My comments: I think it is unfortunate that one of the aspects of using SpeedGrader on a tablet with a stylus, specifically, that it was fast and easy to swipe left, is at the root of the problem here. It is unfortunate that swiping left speedily, after annotating, does not allow SpeedGrader to keep caught up, so losing data. But it is always nice to have an answer.
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