Ah, cool I think you have hit an intended design feature. We use min-device-width to know that you started on a smaller screen (or a wide screen). If you are on a wide screen responsive elements are changed slightly to fit the screen. If you start on a small screen you get the hamburger menu.
Can you file an issue on the github project so that we can work on it to see if it is an issue.
On the homepage, the 'hamburger' menu doesn't show. Every other page is fine though.
On other pages it does hamburger icon properly, so, presumably, it's an issue with that page.