Rebooting your iPhone does indeed mean powering it down completely, and then restarting it. Just “closing the app” doesn’t do quite the same thing, and if it’s an issue with iOS particularly, nothing will change without that power cycle.
Good to know we’re not dealing with the PLAY:1s, that would be a whole other kettle of fish, as they say.
I do agree with @melvimbe’s question as a potential, which would mean that you’re not actually using AirPlay 2 at all, but Spotify’s special system, which is completely different than AirPlay 2.
I’d also be tempted to, at the same time you’re rebooting the phone, reboot all the rest of your network. (again, yes), but unplugging all your Sonos devices from power, and while they’re unplugged, rebooting your router.
However, if you’re not using Spotify Connect, and indeed AirPlay 2 to get the Spotify signal to your Sonos Ones, as I indicated before, that suggests the issue isn’t with Sonos, but somewhere else in your iPhone, since the Sonos would have been proven to work properly with Spotify and AirPlay 2.