Hi ITRTsmith,
With the submission type of student annotations, when your student is submitting their finalized work, it's the same as if the student is turning in a text entry prompt; they can't edit those after they are submitted.
Reassigning the assignment to the student essentially brings it back up on their "to-do list", and ensures Canvas still sends them notifications regarding it still. It does not allow them to adjust already submitted assignments. While it would be nice if they could edit the existing student annotations after reassigning, unfortunately the feature is not that advanced as of yet. They will have to select "new attempt" if they wish to do any additional annotations, and re-add the annotations that they had submitted in their first file.