Yup, having the same problem.
If the menu (WP admin area) is not set, then the “menu_class” is applied to “container”.
In other words, it overwrites the “container_class”.
If the menu is set, then the “menu_class” is applied to “ul”.
I’m guessing this is a bug as it doesn’t make any sense at all.
Ah, its because of the fallback to wp_page_menu function if there is no menu set.
You are right, it’s because of the fallback to wp_page_menu.
But, how can I set options for wp_page_menu when wp_nav_menu falls back?