Yeah it looks like by default it doesn't enable the Submenu, but it can be enabled in Navigation > Site Navigation > Spaces. There you can edit item and check the box for "Show submenu in popup". But it doesn't look like it is working on the 12 Beta2. You can also create your own submenus pointing to categories. 

Will Roberts was trying that but not sure if he got it to work: https://una.io/page/view-discussion?id=6662