major problem with Sonos Arc Ultra - Sonos doesn't let switch to other speakers from TV menu

First of all thank you for providing another data point.  However, my reasoning is a bit different.

1. Even assuming they are not properly adhering to the spect, It would be not possible for large major TV manufacturers to cater to Sonos, since it might potentially break other products, based on the fact that users have been reporting problems with Sonos for years now
2. They have so many TV models they update every year it might  not be feasible to test and deploy it across such a large range
3. They will for sure NOT backport the fix for the older TVs
4. The potential fix might break all the other products that seem to do just fine 
5. Sonos has just a few products so it would be easier to test and deploy it
6. Based on your categorial statements, I presume you have a strong knowledge and familiarity with HDMI CEC protocol
7. “TV automatically defaults to the best sound” QED (The TV’s are  very smart now days and they know where the best sound is; even overriding user’s own choice in TV own menu to switch to say headphones or perhaps some bluetooth or phone; etc;.)
8. Back to reality: HDMI CEC is not just a TV feature per se. It is a protocol that has to be implemented and supported across all the participating devices on the HDMI bus where multiple device can negotiate and communicate with one another with numerous kind of commands and each device is of different and probably can assume multiple types (Playback, Recording, Audio, Tuner, TV)

I still do not see Sonos identifying this a a problem, especially since there is now a solution for headphone users; the Ace.

Especially for something which takes just a couple extra button presses to defeat.

CEC is a parallel buss that is shared by all HDMI connections in the system. It would be possible for a 3rd party (or DIY) device to monitor CEC traffic and issue commands to “amend” or “correct” behavior of seemingly rogue devices. However, this would not stop the rogue device(s) from then issuing their own amendments to counteract these corrections and the system could be trapped in a circular loop.

Another approach would be to insert a “CEC Manager”, similar to a CEC Blocker, into each HDMI connection that is able to manage the two-way CEC traffic for each device. Inappropriate commands could be blocked or replaced by appropriate commands. A major issue with this approach would be configuring and keeping this device up to date with respect to changes made by device manufacturers. The manager would need to be able to deal with past, present, and future devices, plus devices that are not running the latest firmware. Who would keep this device database up to date?

It’s not about the TV, it’s about being able to switch to Bluetooth headphones at nighttime when everyone’s sleeping. I shouldn’t need to buy Sonos headphones to use my Bluetooth headphones on my tv and be forced to unplug my Sonos beam 2 every single time wearing down the hdmi prongs and/or power supply internals. It’s ridiculous. 
 

it’s a simple fix, just let us POWER DOWN the soundbar via the app, so freakin simple.

So what’s the difference between using the app to power down the soundbar, and using a remote to turn off CEC on the TV?  

I have the exact same issue with my Samsung S90D 77”. Sometimes I am trying to change to the TV speakers but a few seconds later the settings will jump back to Sonos system.

Not sure what the purpose of this last post is. From the rest of this thread (and a couple of older threads) it is clear Sonos has had this feature for years and they are not changing it.

I can see this could a problem if you want to use non-Sonos headphones - though turning off CEC or buying a Sonos Ace resolves this. But I cannot not for the life of me understand why so many people buy an expensive soundbar and decide to not use it all the time.

 I know which TV to get now.  A new LG.  My Sony occasionally fails the HDMI handshake which forces me to reboot the TV.  Fortunately that can be done using the Sony remote.  Sounds like the LG would not have that handshake issue.

 I think dintis has the right idea suggesting there may be a way to disrupt the HDMI handshake from the Sonos app.  Not sure this would work though when the TV is LG.  We have an LG in the master bedroom.  I was trying to use Amazon Echo speakers as the sound source for the LG TV.  I worked well, but one of the Echo speakers was having other issues related to notifications.  So I removed them from the LG TV.  As I recall the LG did not automatically switch to the internal TV speakers.  I had to go into the LG menu to switch to TV speakers.  Using a Beam on that TV now.  So it may also be necessary to manually switch the LG to Bluetooth to get that to work.  I’ve never tried Bluetooth on the LG.

 Bluetooth on my large Sony TV requires me to use the Sony remote to pair with headphones.  Is that not possible when using an LG TV?

One button press, versus going deep into TV menu hierarchy using who knows how many clicks.  Last time, I did it, I forgot about and suddenly couldn’t output the sound to Sonos at all, as when the HDMI CEC is shut down, you will NOT be able to play sound through the Sonos soundbar.   I though the firmware update has broken it, spent a huge of amount of time troubleshooting it, until discovering that the option was indeed turned off deep inside the menu.  It is a stupid workaround for the problem that shoudn’t be there.

Sonos has the same problem with the LG.