When trying to select a submenu item in a Quad we need to carefully follow the highlight menu and then select the submenu. If in any moment we move the mouse out of the path it will highlight another menu entry and will close the submenu. This feels buggy and is compounded by the fact the quads are smaller and are used for quick selections. A submenu item selection should behave as the current main top menus in Max. There is a small delay that makes moving the pointer outside not a penalty or a frustrating experience as the quads.
When the submenu is just the nearer one to the menu we could not see the problem. But if the submenu entry we try to use is far on the main menu entry it will close nearly always.
I consider the current quad menus both great and broken. They offer a great functionality but his implementation is flawed because this problem. It is only practical to access to the menus, not the submenus.
Tested in Max 2018