When navigating to the items in a submenu, it is normal to go directly from the submenu header directly to the desired item even if that path goes across other submenus. It is expected that if the mouse passes across those other submenus quickly enough they will not pop up.
On my system, submenus always pop up, no matter how quickly the mouse passes across them. This makes the menus very difficult and frustrating to use.
There is more description of the problem in the KDE bug reports here:
The second bug report includes a picture that helps to explain the problem:
The red line shows the desired path to the submenu item. The blue path shows the way the mouse must actually travel in order to navigate to the item.
(Added by Andy Shaw)
Another approach to reproducing the problem is that you can open the submenu by clicking on the action to cause it, then cut down into the item below and then across quickly into the sub-menu. This will cause the sub-menu to close even though an action in it is highlighted.