Ah alright! I just installed that app and I can hide and show the camera cutout. Have you attempted reinstalling the app? If you can't get it working, this is likely just a beta bug that will be resolved in the complete and stable release.
Go to Settings > Display > Camera Cutout and look for the Apps you dont want to hide the Cutout and tap on Show Camera Cutout all Apps are set to Auto by Default I have to add it still might not show with streaming apps unless you zoom in while watching whatever you are watching but then you don't see the whole scene
