I've enable appenu-gtk from repoitory and it seem that there is a problem to display the menu in plasma bar.
For an exemple if i lauch gimp, then no menu appears. If i click ctrl+o and open an image then the menu appears.
It's the same with libreoffice, sometimes appmenu appears directly, sometimes no.
Don't have much explanation. I search on web and doesn't find ideas for this little bug (no problem with QT apps).