I'll resurrect this as this is the exact issue I'm having. Only on an iPhone X running all the latest iOS. My wife's iPhone 7 Plus, again, fully updated, works fine. My iPad Pro, works fine. iPhone X does the same exact issue as described. I even went so far yesterday to fully wipe the phone. Full factory reset wondering if other apps or MDM (Airwatch) might have impacted it. Set up a brand new phone (no restore), installed the Sonos app first. Used it and the issue immediately happened. Works great until I switch to another app or let the screen go dark. When I re-open it/go back to app, it's frozen. Totally unresponsive and the only way to get it to work is close the app, reopen it, wait about 5 seconds as it 'reboots' itself.