Mine is greyed out, but it seems it's only for apps that are optimized for it. Basically, Facebook and Messenger have been coded and optimized to run in native aspect mode of S8, so it won't let you change it. Not sure why you would want to though
You want to disable full screen because you can't see the buttons for the camera when it's in Full Screen Mode. And now you can't disable full screen. so your only option is to poke and hope that's where the button you want might be.