I used kmenuedit to move menu items around and noticed that when I click on the "K" menu item, then navigate to a particular other submenu ("Internet") as I am hovering over it the menu closes completely. 

This used to happen with my custom "Development" menu used for coding. So I thought it might have been some cruft. So I removed .config/.local/.cache/.kde* from the affected home directory. I re-edited my menuitems as I would like them using Kmenuedit (right-click, edit, on the "K") and it just popped up elsewhere.

This is repeatable each time. 

My mouse is set to focus-follows-mouse. Other menu items work fine, even custom ones now that I deleted my config/local directories and re-did them. It's a completely different submenu that's displaying the same problem.

Steps: Click on K; Select "Internet"; Hover over first submenu next to it "Cisco..."; Move cursor towards first "real" menu item (not submenu); entire menu closes.