I’ve got exactly the same issue. Started this morning and I have tried everything that Seagull99 has done. Same outcome, ie no improvement 

Hi folks, Welcome to the Sonos community, and thank you for reaching out. We appreciate you for bringing this concern to us. We received reports for the same concern and I’d like to share this community thread for your reference. If you need further assistance, I would like to recommend contacting our Sonos Customer Care support team for more in-depth troubleshooting steps or possible product replacement. Please let us know how it goes with our technical support team. We are always here to help.

What’s so weird is that this suddenly started working again for me - and then stopped again! For three days, the Recently Added list for my Apple Music stopped adding any further albums on Sonos, no matter what radical measures I took. And then new album additions started reappearing, totally out of the blue. After another day they then stopped updating again, so no further additions appeared. It’s all very odd. To be honest, I’m getting around the issue for now by just playing my Apple Music to Sonos via the Sonos devices I own that have AirPlay, then grouping these if necessary with those that don’t. It’s not ideal, and it shouldn’t have to be this way, but if I know the Recently Added list will eventually update itself when it feels in the mood then I guess I can live with it in the short term.