Menu Customizer UI/UX
In the menu customizer, submenus seem to be on the same level or even one level above the actual (main) menu item.
It can appear that way because
1) "(Submenu)" is written in bold font, while the main menu item is in regular font.
2) The "(Submenu)" item is listed on the same level as the main menu item, or can even appear to be above it because the arrowhead is left of the icon of a main menu item.
Indent the submenus towards the right (including the arrowhead) and perhaps using bold font for the main menu items while keeping the submenu item(s) in regular font.