I implemented a fix to the symptom but not to the problem. I found that the site was displaying which every menu appeared first in the list of menus as the primary navigation. It was ignoring the primary navigation checkbox.
So I renamed the main menu to “1 – Main Menu” so that it appeared at the top of the list. This resolves the problem on the surface but not the actual underlying cause.
My apologies that you were seeing the menu after I figured out this fix.