It looks like a bug but I am not sure which program is to blame..
In Plasma 5.12 the option to add global menus in window titlebar has been removed. There is a panel widget that adds global menus to the panel, but also global menus are still available in the window titlebar as a button. I have added that button with global menus as indicated here. That’s under System Settings > Application Style > Windows Decorations > (tab) Buttons:
The idea is to get the global menus into a left-side button on the window upper margin; like for example in Chrome:
When this is done, Libreoffice has no menubar anymore. Adding the Menubar button and using it has no effect.
But once the “Application” button is dragged away from the titlebar at System Settings > Application Style > Windows Decorations > (tab) Buttons, the menubar in Libreoffice comes back.