Tablet pointer (usb Wacom Intuos Pen & Touch) shows and can use 1st order menus from panel (favorites) . Clicking on a category (internet, multimedia, system...) shows normally the respective submenu. But hovering on this submenu lets it disappear. No problem with standard usb mouse. (KDE 4.14.25, plasma 5.8.2-1, Linux Manjaro stable)
With a tablet pointer we get a hoverEnter when we press the pen and a hoverLeave when we leave the window but not when we leave an MouseEventListener and not when we enter. This means when we click on the first menu, that takes focus, it gets contain mouse, we move over the submenu - the first menu gets a mouseLeave and because we haven't pressed the pen, the submenu doesn't have focus so everything closes.
This appears to be working in Plasma 5.25 on X11 and Wayland.