I had the Sonos speakers setup via the Labs integration before, and then migrated to the LAN Player Websocket thing via the IDE when that change happened. I migrated to the new app, and the speakers stopped working via Webcore.
I checked the new app, and they show offline. They are most definitely not offline, as I am able to play music on them via Alexa, so Alexa is able to connect to them just fine.
I therefore removed the speakers, so I can re-add them. Unfortunately, now they are not being discovered by the new app at all!