That’s because I believe Apple Music is the only service which will play through the AirPlay device with your iPhone being simply a remote. Every other service is simply cast from your phone (plays directly on your phone) so closing the app terminates the streaming. It’s definitely inferior to Chromecast and Spotify connect in the way this works but it’s just Apple’s closed ecosystem
That’s because I believe Apple Music is the only service which will play through the AirPlay device with your iPhone being simply a remote. Every other service is simply cast from your phone (plays directly on your phone) so closing the app terminates the streaming. It’s definitely inferior to Chromecast and Spotify connect in the way this works but it’s just Apple’s closed ecosystem