My phone did the same after the first patch for Samsung one ui android 9. I loved the full screen gestures and now my quick settings has the navigation bar setting to hide it but it's always half lit and you cant turn it on or off.
I think I found the problem as this was bugging me to no end. It seems to be related to the "Accessibility" functions. Try this Settings>Accessibility>Installed Services>Accessibility Menu: Turn OFF. Then go back the to Navigation Bar Settings and you should find the full screen gestures option available again. The Navigation Bar the Notification Menu Buttons drawer should also work to turn this on and off.