I use my AirPods Pro every day. I have learned that when out walking it is easy to just say “Hey Siri, turn on noise cancellation” and conversely to say “Hey Siri, turn off noise cancellation”.
Variations also work like “Hey Siri, turn noise cancellation on” or off"
Since installing iOS 15.6 asking Siri to turn ON noise cancelation still works. But asking her to turn it off, fails every time. (“I’m working on that”, “try again later”, etc.)
I have Beats Pro and don’t have that problem (but I always press and hold the buttons to change modes), but I get those Siri issues when I use my watch occasionally, and a restart always fixes it. You can’t restart the AirPods, but I’d try restarting the phone.