In my experience, this issue comes up every time a student hands in a submission individually on a group assignment. It looks like the comment just dissappears in Speedgrader, but in reality the comment will be saved and available to the student. You can also find the comments if you check via "Grades", but in speedgrader it looks like they are not saved.
A workaround for this is to check the "Assign Grades to each student individually" in the assignment setting, but that will create more work when grading the group submissions.
Again, this is only a issue when a student hands in a assignment individually on a group assignment without being a part of a group. We have been using Canvas since 2017 and this "bug" has unfortunately always been there.
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