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’
