There have been at least 2 threads since the release of opera 55 that the issue is discussed. In short, we named in "the ugly csd and its side effects" 😛

In other words, what you now see when right clicking is a menu made by opera, not the menu of your window manager, because opera uses its own buttons for minimize/maximize/close.

p.s. Opera was updated to v56 today and the issue remains unsolved, so it will probably bug us all for some time.