Since version 5.8.95, and after the desktop is running for some time (order of magnitude 1 hour or so) the Application Launcher sometimes stops appearing. The launcher button shows the little blue bar on top when pressed (using the breeze theme).
Not seen many other reports of this. Can you run plasmashell from a command line, and then include any output. Are you switching between monitor setups?
Actually this bug can be closed, it seems to have disappeared with one of the point releases (even before I noticed my mesa problem).