All pages with any page sections on them are active links on the menu. This means that when you click on them, the browser will load the page.
On the desktop, the subpage menu automatically opens when hovering over the top-level page. On the mobile, the submenu only drops down when clicked because hovering doesn’t work on mobile. But when clicked on the mobile, if the page is an active link, then that page will be loaded, instead of the subpage menu being displayed.
The solution is to move page sections from your top-level pages to subpages. This essentially makes the top-level page blank and a suitable container page for subpages.
Please note that when logged into the Site Manager all pages are clickable so blank pages can be edited.