I’m not exactly sure what problem you are having unless the presented view controller has transparent parts where you are expecting to see the presenting view controller behind it. A full screen presentation is supposed to cover the entire screen, so iOS will actually remove the presenting view controller’s view from the window while the presented view controller is visible. If this is the problem, you can use an overFullScreen presentation style to stop iOS from removing the presenting view controller.
Full screen is the default before iOS 12 so it should work the same as it always did when using that style on iOS 13. When you dismiss the presented view controller, the presenting view controller’s views will be added back to the window and you should see it again.