Hi there,
the Theme simply registers a Navigation Location, it is a core WP function ( via the Appearance > Menus ) that sets which menu is displayed. And we do not interfere with that function. So no, its not a bug in the Theme.
First thing to try is:
1. Go to Settings > Permalinks and Click Save Changes to Rebuild the Permalinks.
2. Clear ANY caches.
If the issue persists then check for plugin conflicts by temporarily disabling all other plugins.