I think I might have found 1 possible cause: The Main Nav Width (default).
I changed things around to center/center (logo/nav) and as soon as it snaps into mobile layout (exactly 768px wide) the navigation elements go into “hamburger icon + search icon”. However that is when the extra width is showing again.
I’ll keep digging but I haven’t added any extra styles (definitely not padding) to the right/left of these elements.