I use the Sydney Theme and have the problem that I do not get a drop down menu on a large iPad.
For small tablets the theme changes to the Mobile Menu. On large tablets, the desktop menu remains. In this I do not get a dropdown menu when i tap it. As soon as I log in and cover the site everything works fine. Other themes like twentyseventeen also works fine.
On a new Wordpress installation without plugin, the same error occurs.