Hello Andrii,
Although the Gallery section of the one page is now showing up in the menu I am still not seeing the about page which is not included in the one page website, but is not excluded from the menu. the only page excluded from the menu is my ‘short about’ page which appears at the top of the ‘one page’ page, so this is working correctly.
Essentially the problem now seems to be just for the menu item to link to an external page.