Ever since TVOS 14, volume control has been very strange on ATVs for me. Previously, I could only use the +/- buttons to control the volume of stereo setup HomePods connected to the ATV. Now anytime I try to either adjust the volume of the TV speakers or the receiver. If I went to Settings> Remote Controls >> Volume Control, I can turn it off, which is fine, but it still does not adjust the HomePod volume.
The audio output is set to HomePods. Navigation clicks, movie trailers in the TV app and everything that is not in another app will only come out of the TV or receiver even when the output is HomePods. When I start watching something (like on Netflix, HBO Max, ATV +) the sound comes out of the HomePods, and I can control the volume on the HomePods.
Edit: When I posted this I was on TVOS 14.0.2 which came out today. I saw another post that seemed similar, and someone said that 14.2 beta seemed to fix this, so I updated, but no luck for me. To give you an example of strange behavior. I scroll around the home screen, there are no clicks. I open the Netflix app, I do not get the opening DUN DUN sound. I browse the Netflix app, no clicks. Through all this, if I try to adjust the volume, it adjusts the TV / sound line. If it turns up loud enough, you can now hear the clicks, because everything that is not actually media, just comes out of the TV / sound field. SO when I play a show, the sound comes out of the HomePods, and I can adjust the volume of the HomePods.