Cannot adjust volume with buttons during FaceTime "ringing" - iPhone

When I am calling someone on FaceTime, during the "ringing" phase, the volume button do not work. If I want to reduce the volume of the ringing, I have to go into the control center and pull the volume slider down for instance.

I think this is a bug, the volume button should definitely be working.

iPhone 15 Pro Max, iOS 18.7.2, I have noticed this issue since at least 1 year.

Similar to this: https://discussions.apple.com/thread/255760458