Full Screen while "Displays have separate Spaces" is disabled.

Full Screen is useless when I have "Displays have separate Spaces" turned off. When getting an app to Full Screen in one display, all the other displays show a black screen. I can't use other apps while having a Full Screen video in one of the displays, for example. I understand that when an app is Full Screen, the app is turned into a Space itself, but still, MacOS should have an exception for when "Displays have separate Spaces" is turned off otherwise Full Screen doesn't make much sense when combined with that system configuration.