After upgrade to Kubuntu 18.04 I'm observing a strange behaviour of the main menu. When it's opened for a first time it automatically switches to the last tab (where Reboot/Power off/'Switch user' items are stored) instead of the first tab (Favourite apps). That happens only on a very first opening after new Plasma session is created. After that the right tab is always shown.
*** This bug has been marked as a duplicate of bug 391432 ***