Currently when you have a show paused inside an app (like Paramount, Roku Channel, PBS, AcornTV, etc) the screensaver you have selected in Roku Settings will not show. It will instead show the Roku Rain screensaver. Apparently this is by design to conserve resources.
However, in most modern Roku devices there is plenty of RAM to allow a screensaver to show over the top of a paused stream. See https://developer.roku.com/en-gb/docs/specs/hardware.md
As you can see from the chart most devices now have 1 GB and more of RAM. That should be plenty of resources to allow showing the selected screensaver when shows are paused inside apps.