This first came to light with the Menu-bar missing in LibreOffice I then found the Menu-bar missing in quit a few other applications.
I found the cause was the KDE Application menus daemon
To fix the issue you need to go to System Settings -> Startup and Shutdown -> Background Services, and uncheck Application menus daemon Apply and reboot.
I had the same issue, but Background Services no longer appears in System Settings. Filed a bug at KDE - the helpful folks there provided a simpler solution:
Go to System Settings > Colors & Themes > Window Decorations > “three dots” icon > Configure Titlebar Buttons
2. Drag “Application Menu” item out of the preview titlebar area
3. Reboot
In current System Settings (Plasma 6.6.4) it’s actually found at :
System Settings → Appearance & Style → Colors & Themes → Window Decorations → Configure Titlebar Buttons
Welcome to the 
-Forum. Keep in mind that thread is 4 years old. A lot has changed.
I close it right now. If you have any issues please open a new one