This is apparently due to the new AirPlay functionality. Control Center stops listening to those ports when I turn off “AirPlay Receiver” in the “Sharing” System Preference:

For some reason this doesn't work for me when I switch to the macOS build. [The documentation][link] lists macOS 11 support on the website (and the docs in Xcode list 10.16), but I'm getting an error that my view "has no member 'onOpenURL'". [link]: https://developer.apple.com/documentation/swiftui/anyview/onopenurl(perform:)