Hello Gregor-1138,
Thank you for posting in Apple Support Communities. We understand that you are having some issues with your Apple TV remote controlling volume and mute functions since the last firmware update. We would love to provide you support on this.
There may be confusion between the system of each device. In some cases, when a product updates, the other product that it connects to also has to be updated (in some cases not all). Until then, the products no longer communicate effectively. Which is what you saw when you used your old remote that has no update and it worked great.
-For example: When the macOS updated from Catalina to Big Sur most HP printers stopped connecting and working with Mac computers until they received the update from their manufacturer. Also, although this isn't your situation, we just want you to understand how another manufacturer's products can sometimes lose communication due to updates.
-We would recommend disconnecting the receiver from your TV and unplugging its power, then reconnecting and setting up as new to see if that resolves. If not, reach out to Pioneer customer support or a device forum to see if there is a current software or firmware update to resolve the issue of miscommunication.
Thank you for allowing us to support you.
Have an awesome and productive day!