I have this exact problem but I am using a Samsung QLED. I am able to turn ON the TV but the OFF command gives the chime as if it worked, but the TV remains on.

Found a workaround for this issue… It might work for your case if you’re using Alexa inside the Sonos ARC.

I named the Sonos Arc tv control device “TV”. This can only turn ON the TV due to some limitation I have yet to figure out.

I created a second Alexa device called “Television”. This can only turn OFF the TV.

So to turn on the TV I use “TV” and to turn off the TV I use “Television.”

Hi @lukeremis, Thanks for reaching out, thank you for sharing this information. I hope it will help other Sonos community users in the future.

If there's any other issue you're having in the future, feel free to reach out.  

The Sonos Community is always here to help.

Hi @giora, Thanks for reaching out and welcome to the community. Regarding your concern about your Sonos Arc, you may try the suggestion of @lukeremis to check if that will work for you. If not. then I’ll ask you to submit a diagnostic report for us to investigate further what could be causing this issue. You may reply to the thread with the confirmation number to assist you with this issue. 

Let us know how you get on with the advice above.

We're here to answer any further questions you have.