Slightly different problem here. My setup:
ATV4 <> Sony HT Receiver via HDMI <> Toshiba via HDMI ARC
Power on and off works fine, but unpredictably, the audio sets to TV instead of Receiver. Have to use the TV remote to go into the menu and change the audio output to the Sony receiver.
Also, if I leave the ATV on screensaver with no audio, the Sony shuts down after 20 minutes or so, ok if playing audio such as iHeart\Pandora\iTunes.
In my case, I think the Sony receiver is part to blame, no issues with Onkyo until the HDMI interface board gave out on it. But, it seems ATV's HDMI-CEC is perhaps not completely compatible with all of the component CEC implementations. I understand the CEC standard is kind of a loose collection of compatibilities and perhaps every component manufacturer does their own thing to a degree.
Only suggestion I have is to double check the TV settings. My Toshiba TV has a setting to enable the CEC capability, but many manufacturers call it something else. I think LG calls it SimpLink. I recall there were also some settings having to do with power up\down in the TV setup as well.
Update: Thanks to your inquiry, I took another look at my Sony receiver and found the settings that fix both the sleep issue, and the audio switching on the TV. But, again, in your case, the TV is likely the culprit as you have ATV <> TV <> Sound Bar.
