Hi everyone, thanks for raising this with the Sonos Community!
I’ve not been able to reproduce this on my own system with YouTube Music, and it looks like you’ve done some troubleshooting steps already.
Have there been any recent changes to the network? This can sometimes happen if mesh networks are in use, due to Double NAT - if you’re using a mesh network in conjunction with an ISP provided router, it may be worth either placing the mesh network in Bridge/AP mode, or the ISP router in to Modem Mode.
Otherwise I’d recommend getting in touch with the Support Team via either live chat or phone call, as they have more tools available for investigation :)