It seems there's conflict in the "Media query for iPad and tablets" rule and normal stylesheet, or even with the theme.js; when testing the theme (including the official demo version online) on specific devices like ipad, start with landscape mode then change to portrait mode, somehow the site-navigation div, which contains all the menu items, became empty.
No menu items, no element style, hence the whole navigation menu disappeared, all left on the first sidebar were only widgets.
Not just on my test page, also happened to the official demo page of this theme.
I tested on other handheld devices too, with different browsers like chrome android, same problem occurred.
To avoid this, I had to start loading the website with this theme, on portrait mode first, then the theme would work perfectly fine. You can turn your device anyway you want, the menu still there, responsive effectively.
Hope this one will be fixed soon, other than this, products from this developer are all good stuff.