gnome-terminal keeps the menubar even when the profile is configured to hide it
Solution 1:
In my case it was related to Eliminate the top "mac-ish" file/edit/view/... menu after installing gnome?
When I moved my top bar away with Alt + Right click (for example to the right side of the screen) an extra menubar showed up on the top of the screen. It was probably hidden under the top bar I moved away.
After running sudo apt-get remove appmenu-gtk3 appmenu-gtk appmenu-qt and a reboot gnome-terminal --hide-menubar worked again :-)