I have also this issue with a new website upgraded from D7. When opening a submenu, there is no way to go back to the main menu but to refresh the page. It's a minor annoyance but could be a bit confusing to the user.
Fix suggestion #1: When closing the menu with the X, opening it again would open the main menu instead of going back to the submenu. This to me is the intuitive behavior I would expect.
Fix suggestion #2: Otherwise, add a Return button.