any libre office window should not be put in full screen mode, otherwise it can't be brought back to the windowed mode, there is a black line on top instead of the regular menus, and a black screen appears amongst other fullscreen apps.
this is not new to the LO version i'm using, neither the mac os 10.11.6
Exactly, it is a duplicate of bug 115284
*** This bug has been marked as a duplicate of bug 115284 ***
Or given that you mention you are on macOS 10.11, more likely a duplicate of bug 72490, but a duplicate nonetheless.