Double line on mobile menu

Hi, the main horizontal menu in my header works just fine on desktop, but on mobile devices, it shows as two lines, one below the other. I have it set up as a menu module within the header section.

There’s only one item in the mobile menu (the hamburger) and when I have it set to be centred, it shows up on the right edge of the top line — i.e., right in the middle of where the menu bar would be if the top and bottom lines were joined.

Unfortunately, I can’t share a URL and I seem to be unable to attach a screen shot. I’ve tried using the various settings panels with no luck. Thanks in advance for any suggestions.