I’ve tried your code and it’s working as expected.
How many other things are appearing in the admin menu between pages and comments? If you have too many and there is a conflict items with the same menu position will not be shown. Pages is a 20, comments is at 25, this means if you have more than 4 menu items additional menu items at position 20, then some may not be shown.
Try setting the menu position to something like ‘20.111111’
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users