The "comment" text field is just for keeping an internal description of the expression and won't be shown outside the object properties.
Please check the "Text as Pop-up" option for each expression. If you need to show something different from the result, a valid solution is to add extra expressions with only that property selected, so the rest of the chart remains unaffected. Take into account the "Pop-up labels" check under Presentation tab.
This is the thing, when I hover over the column headings (the title is abbreviated, hence the comments for the full description) the comments box appears but not comment in there. I think I may be missing something in the comments property around the full description for this to be recognised and picked up in the comments box when hovering over the column title.
I've tried a comment for the abbreviation AMH i.e.
1. ='Adult Mental Health'
2. ="Adult Mental Health"
3. 'Adult Mental Health'
4. "Adult Mental Health"
All the above show the comments box but no comments are shown.
Hope this explains a little better than my original post.