Volume only controls one HomePod from my stereo pair with Apple TV
I have two 2nd-gen HomePods from Feb 2023, both up to date, with an Apple TV 4K bought at the same time and also up to date (tvOS 16.6). Have had them working together as a stereo pair and home cinema output with eARC since release with no issues. It's been great up until this morning when suddenly the volume control on the Apple TV remote would only raise or lower the volume on one side, with the other side staying at a fixed volume.
This appears to be an issue that several people encountered on 1st-gen HomePods and on minis but haven't found anything about 2nd-gen HomePods. I have tried everything suggested on all posts I have found, to no avail. This includes ungrouping HomePods, restarting them, unplugging, factory resetting, pairing them again, restarting Apple TV, etc, all many times in various orders. The problem remains. I have succeeded in getting them to function as a proper stereo pair if they are simply an audio output for my Apple Music via iPhone, but as soon as I use them as the Apple TV audio output (with or without eARC), the issue reappears.
I also only have have one WiFi channel (2.4 GHz) and no 5GHz. Apple TV is connected via Ethernet.
I have tried everything short of factory resetting my Apple TV as I don't wish to lose all my data, settings and apps. I have also not tried to Remove Home from the Home app, for the same reasons.
Any suggestions would be welcome.
PS: I must also specify that another weird Apple TV volume issue appeared at the same time and is possibly related - when I change my Apple TV audio output back to TV Speakers, the remote volume buttons act as though they are held pressed in. One quick press on Volume Up will send my TV (2018 LG) volume scrolling all the way to 100 and same down all the way to 0. Cannot be overridden by the TV remote unless I deactivate Volume Control in the Remotes and Devices settings.
Apple TV 4K, tvOS 16