"Floating" menu shows on mobile devices

I spotted an issue when our site is viewed from a mobile device. Apparently, everything looks fine, but, if you swipe to the left, you will see that the dropdown menu, which should only be visible when the “bars” button is touched, is appears as a vertical column on the right, while it should stay completely hidden (see screenshot). Important: this issue is only visible on mobile devices, shrinking the browser window on a desktop won’t show it.

I tried removing all custom CSS and also installed the theme on a clean website, but the issue persists, therefore I reckon it’s due to the theme itself. Could you please give me some indications on how to hide the menu correctly, without breaking anything? Thanks.