Seems a lot like a CEC implementation issue with the Sony, or a failure upon your specific Beam, since this behavior is not common across all Beams. I’d recommend three things.
First, try checking with Sony to see if there are any software updates to the TV, and apply them.
Second, reboot the TV by unplugging it from the wall for two minutes, then plugging it back in.
Finally, if neither of the above processes fix the issue, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this restart issue, and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.
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 phone folks, there are more options available beyond just the diagnostic analysis.
Thank you Bruce, I will go through your recommendations and come back if it continues.