Cannot Edit Notes in MS Excel for Mac

With Excel 365 on Windows I periodically have similar problems. Although the cell-marker for the presence of a Note remains unaffected, Notes may not display on_hover, the option to edit or delete a Note may not be offered, a Note's display_frame may show but with no content or colour yet blanking the covered area of the sheet, exercising the Review tab option to Display all notes has no effect - though this may have some self-limiting setting (for which I haven't checked) when there are a great many notes. The problems are not confined to a particular worksheet or workbook, and don't necessarily affect all open sheets or books. I've never found a cause. However, it's usually the case that if I close all instances of Excel and re-open the offending worksheet normal behaviour returns. On occasion I think it may have been necessary to restart the machine - but I haven't stopped to log details.