Multiple apps (e.g. gwenview, Konsole, System Settings, Dolphin) seem to have their own setting remembered in local configuration, so if they have a single run without menu, these tend to remember "menu disabled" setting.
TLDR: look for
MenuBar=Disabled
and change to
MenuBar=Enabled
in ~/.config/ of those apps (when they are closed!).
Details: I found a great explanation of that behaviour here, thanks to Demis Palma.
My personal opinion: I wish KDE designers would not follow harmful trends from less professional desktops as this introduces mess like that. Each app should have all its belongings inside its window - the concept of looking for a part of an app (e.g. global menu) in some detached place should be left in the past (Amiga Workbench!) or as unsupported hobby project.