Hmm, what window mode are you playing in (windowed, fullscreen, borderless), and what exactly happens? Does the OS capture a empty image, or does it look like the game is intercepting the screenshot shortcut before the OS receives it?
I'm inclined to say that that's an OS problem rather than a game problem, since screenshots seem to be behaving fine here on Linux, but if we can get a little more information, we might be able to come up with a workaround and/or implement an in-game screenshot funcion.