I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution. Especially about what device may be creating the CEC command to shut down the TV.

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.

Since the shutdown is regular at 30 minutes, I suspect that one of your devices is in “power saver” mode and will shut down if it does not receive any operation commends in that interval.

You can explore this by giving a nonsense command to a device at about 20 minutes. Identifying which device is giving the power down command may not be straightforward. Possibly there will be a brief message displayed that you can correlate with a specific box.

Another approach to identify the device issuing the shutdown command would be to insert a CEC blocker and systematically move it between devices. When you insert the blocker in the device issuing a system wide shutdown command, the TV will stop shutting down. However, you should consider the possibility that the is in power saver mode or that a box is shutting down without issuing any CEC commands to the other components. If this is the case, the blocker will have no effect.

Another approach: remove all HDMI cables from the TV and power down the TV and soundbar for at least two minutes. Next, power up the TV and soundbar, but don’t connect the HDMI cable. After the TV and soundbar ar finished rebooting, connect the HDMI cable between them. They will discover each other and work out a compatible communication scheme. I can’t guarantee that this will cure a power save setting in one of the boxes or TV.

Go through the Xfinity setup in detail. A firmware update may have inserted or changed a power save setting.

 Is the TV actually off or is it just the XFinity cable box that shuts off?  I have XFinity and a 3 year old Sony TV.  My issue is the opposite.  The TV occasionally turns itself back on.  The turn on does not always happen nor does it happen at a predictable interval.

Tried all of that. Was on the phone with 2 different technicians and 3 different DX tests, who both reset everything from their end, and it still shuts off. 
It only happens when the mini sub is connected. 

Which model TV? Is the TV capable of being controlled by network commands?

I'm curious about the Mini connection. What happens if you have the Mini powered down as you start a TV session, then power up at a later time? Does the 30min clock start at power up?

I’ll have to try the power on option for the mini tonight. 

BTW it’s a Samsung 75” Smart TV less than a year old I got at SamsClub.