I have 2 HomePod Speakers in Stereo. However, my problem happens even if they are not grouped.
I have my Apple TV audio directed to play on the 2 HomePods (just selecting the group). And this works very good. No problems with any apps including Channels.
But, when I SLEEP the Apple TV and then restart the ATV the Channels App will start playing on the TV Speakers instead of the HomePods. Even though the Apple TV Audio is still set to the HomePods. And if I go into any other App (I tested about 8 of my most used apps) the sound comes from my HomePods. And then back to Channels and the sound comes from the TV Speakers.
Then, if I simply reset my Apple TV Audio to the HomePods (i.e. deselect them and then select them) things are back to normal with Channels Audio going to the HomePods until I SLEEP again.
I am using tvOS 12 on a Apple TV 4K and using a 4K VIZIO TV.