Disconnect everything from the TV including the Beam. Unplug the TV and Beam from power for a couple of minutes. Plug the TV and Beam back in but keep them disconnected. Be sure VIERA Link (CEC) is turned on in your TV settings. Run TV Setup in the Sonos app, and when the app instructs you to, connect the Beam to the TV’s HDMI ARC port using the Sonos-supplied HDMI cable. When the Beam has been successfully connected, connect any other device to your TV.

Make sure HDMI-CEC is enabled on the TV - Panasonic call this either VIERA Link or HDAVI Control or EZ-Sync and so just ensure it’s switched on in the TV settings.

It’s more likely an update on the Panasonic TV of some type. It’s not clear if you’ve unplugged the power on just the Beam, or also the TV, but if not, I’d certainly do the TV as well, to force a reboot of its operating system and CEC/ARC, something that doesn’t occur when you turn it off but leave it plugged in to the wall. You might be able to prove if it’s the ARC connection if streaming music directly to the Beam works, but it just won’t play the TV’s connection.

I’d also suggest before returning the Beam, you just try a call to Sonos Support directly to discuss it. They could, if you were to run a diagnostic, be able to tell if there was a hardware fault in the Beam that could be causing the issue, and if so, issue a replacement to you. 

There’s lots of opportunity for guessing here on these forums, but only Sonos can read the data in the diagnostic you might submit, which would hopefully answer a lot of questions.