Recently the Siri Remote got an over the air update from Apple TV 4K. The version is indicated on Apple TV > Settings > Remote > .. Version 0x0070.
Since then the volume control (louder, quieter, mute) was not working anymore. I guess thats happened due to changes in the HDMI connection protocol or something.
To fix this
- disconnect TV & Apple TV from power
- disconnect the HDMI (required)
- wait for few minutes ☕️
- connect HDMI
- power on
The fixes at Apple Support did not help.