We are all saying the same thing!
We know that all the content Channels displays is SDR so the original choice by the developers was to switch the aTV to SDR on app launch rather than per video to stop multiple switches in browsing the guide and channel hopping for example. The aTV then switches back to the users choice when the app quits or she changes to another app or the home screen.
Apple provided a way to do this but it cannot be switched off in the aTV Settings because it does not respect the Depth/Framerate toggle.
Some people get HDMI handshake issues with their setup when switches occur and need to turn off depth changing (or turn their TVs off and on again in my case). They can then either stick to HDR or SDR whichever they personally prefer.
Either Channels has to change to the swap per video option which will produce lots of switches but can be turned off if need be or Apple need to make the per app setting respect the user choice.