Seems odd. The ARC settings on the TV should be ‘on’ all the time. I’d be tempted to check the TV for updates, and apply any you find. After that, I’d reset the OS of the TV by unplugging it from the wall for two minutes. 

If the issue continues after that, and if the TV’s support group can’t help, 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.

Awesome!  Thanks Brice - i will give that a t ry

Also make sure Bravia Sync or Bravia Link or Control for HDMI (whichever applies to your tv model) is turned on to automatically recognise the HDMI input of your Arc when the TV is turned on.

This is a commonly reported issue with Bravia TVs.

This has been happening with my Bravia TV since the last Sony firmware update. Prior to that it worked flawlessly for 14 months. Sony deny any issue exists.

In general, TV’s and soundbars sniff out the capabilities of the other guy at a fresh boot when nothing is connected to the TV except the soundbar. Boot both items before connecting the HDMI cable. SONY TV’s won’t usually reboot after a simple power interruption. I don’t recall the exact procedure, but it can be done with a multi-button press on the remote. SONOS soundbars will reboot after a simple power interruption. Some TV’s require a minimum amount of power down time before they will reboot. (SAMSUNG is two minutes) At this time it’s a good idea to make sure that the TV will communicate its details over the HDMI connection. ​@Rhonny gave details above for the SONY’s.

After the reboot, connect the HDMI cable between the two devices and allow a couple minutes for the units to exchange capability details. In my experience this is the user’s best shot at establishing proper communication between the units. Now you can connect other devices to the TV. Other devices, particularly games and AppleTV may attempt to control the system and can cause issues by sending CEC (Consumer Electronics Control) commands that can cause odd input selection, audio, and video issues. If a device is asserting itself with unfortunate CEC commands, it can be isolated with a CEC Blocker.