AppleTV CEC volume control via iPhone/iPod

AppleTV 4K, A/V receiver. No issues with full remote functionality using iPhones and iPods—including volume—for more than a year. Recently, volume control stopped working on all iPhones/iPods while other functionality continues to work. AppleTV connection to receiver has not changed (same input). Only thing that has changed in our setup is that we have a new TV. As I understand it, the new TV should not have affected phone remote functionality since the AppleTV is directly controlling the receiver volume setting. Is that correct? Is there a reason the TV might have affected this? Did a recent iOS update possibly cause this? Can you please offer suggestions for settings to review or other potential resolutions? Thank you.