Menu Bar Popping Up

I am using the free version of Make.

I have positioned the navigation menu at the top of the header area.

When viewing the site on a mobile device, not only is the “hamburger” menu not being used (the menu is arranged above header), but when one of the navigation menu tabs is clicked, a small bar with the “hamburger” menu on it pops up very briefly (approx. one second), just below the header; then, it – the bar and hamburger symbol – disappear.

I have been using the Make theme for a few years, now, and have never encountered this issue.

I hope that someone can help me resolve this.

The page I need help with: [log in to see the link]