Since ‘all other lossless services perform normally’, my supposition would be a bad DNS to the Apple servers. Try a reboot of your router, followed by a reboot of your Sonos devices, to pick up the newest DNS information, and test.
If that doesn’t work, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, 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.
OK thanks - it does seem strange that this issue is isolated to Apple Music
What Sonos speakers do you have?
And what other lossless streaming services do you subscribe to that are working normally?
Which is why I suspect a DNS issue, which could apply between your speakers and the Apple servers.
If it was a local issue, or a Sonos software issue, it would apply equally to all streams.
But, as I suggested, to be certain requires you to contact Sonos Support.