On the computer version, adding comments to a pin is straightforward.  You write the comment, type return, Grading on my iPad for the first time, I used the "pin" annotation, wrote my comment in the comment pane for each pin, clicked on the big "Done" at the top of the commenting pane — and then moved on.  Because I did not know that one needed to manually save the text first by clicking on the up-arrow button (which was not even obviously a button), every comment was lost.  I only learned about this when students told me,

Minimally there should be a warning when clicking "Done" and the text field contains unsaved material.  

(as discussed here:  https://community.canvaslms.com/t5/Question-Forum/Canvas-Teacher-comments-disappear-with-no-error-me...)