Not experiencing this issue at all, on any of the three iOS devices I use regularly.

How did you determine it isn’t a network issue? It bears all of the hallmarks of one, since your controller you’re opening is on a different networked device than the Mac. It certainly seems to me that your iOS device is having issue communicating with the speakers, whereas your Mac is not. 

I’d be tempted to power off the iOS device, then reboot the router. I’d wait two minutes for the router to come up, then I’d boot back up the iOS device.

Alternately, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, from the iOS device, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks for the suggestion, Bruce. Restarting the router and phone did the trick. Who would have thought the “switch it off and switch it on” would work!

If that had failed, we might have had to resort to reading the manual!!!!