I think I have fixed this, at least in terms of "Comments" vs "Notes". I can't comment on any Mac related issues as I am using Excel under an Office 365 subscription using Windows 10.
I had created my Comment by right-clicking on a cell and selecting to add a Comment. Whenever I had done this previously, a yellow box would appear with my username at the top to which I could add my text. Then, once created, I could permanently Show or
Hide this using a new menu option which now appeared when right-clicking the relevant cell.
I did the same this morning, but the yellow box didn't appear. Instead, you get the style of comment which you see in Word, in a grey box with a text field.
After some exploration of this thread and looking at my version of Excel, Microsoft appear to have renamed the older yellow box style "Comments" to "Notes".
If you create a "Note", you will create the yellow box we used to understand to be a Comment which can then be permanently shown or hidden.
Hope this helps!
Neil