In Moodle web, create an assign. The submission type doesn't matter. Login as a student and submit the assignment (web or app, doesn't matter). Now login as a teacher in the app and go to the assign. Select "Participants" and then select the student of the previous step. Go to the "Grade" tab and click to edit the "Feedback comments". Input something in the comments and save them. Back in the "Grade" tab, input a grade and click "Done". Check that the grade and the comment were saved. Now click to edit the "Feedback comments" again, change the value and save them. Back in the "Grade" tab, click "Done" without changing the grade. Check that the comments are updated.