Best answer by Jeff S
Welcome to the community. We're investigating an issue with playback from the storage on iOS devices when there are non-Sonos AirPlay devices on the network and your iOS device goes into auto lock. This is likely related to the recent AirPlay 2 implementation. We'll update this thread when we have more to share from our investigation, but for the meantime you have a few options to get music streaming again.
First, you can disable AirPlay on any non-Sonos device on your network. Another option is to disable auto lock on your iOS device in its settings menu under display and brightness. A third option would be to sync your music to the iTunes on a computer, then set up a music share. This will allow your Sonos system to play your music library even when your phone is locked, AirPlay devices are online, or if your iOS device leaves the network.