If you override a grade in the gradebook including an alteration to the feedback then un-override the grade the grade reverts to the grade in the activity. However the altered feedback is retained.

If you then make any further alterations in the activity itself the updated feedback is overwritten by the feedback from the activity.

We either need to copy feedback from the gradebook back to the activity (not sure this is a good idea) or revert the grade AND the feedback when the overridden checkbox is unticked.