Agree with you @conn that what you request makes 100% sense and I don't see the use case for this "feature/oversight" neither. Don't have the Audio Pro but long time Sonos (Ones) stereo pair user.
Maybe try to make the use cases and expectations clear between 2 different speakers in few scenarios. My understanding of what you report.
Use case 1: 2 speakers, 1 or 2 rooms, not stereo paired, not grouped: control each speaker volume separately both from app and physical buttons. Working OK.
Use case 2: 2 speakers, 1 or 2 rooms, not stereo paired, grouped: control each speaker volume separately both from app and physical buttons. Working OK.
Use case 3: 2 speakers, 1 room, stereo paired so it should become seen as a single speaker, potentially grouped with other speakers but irrelevant: control speaker volume stereo paired speaker set as one from app working OK but physical buttons still only adjust volume on a single channel. Definitely NOK.
Maybe try to make the use cases and expectations clear between 2 different speakers in few scenarios. My understanding of what you report.
Use case 1: 2 speakers, 1 or 2 rooms, not stereo paired, not grouped: control each speaker volume separately both from app and physical buttons. Working OK.
Use case 2: 2 speakers, 1 or 2 rooms, not stereo paired, grouped: control each speaker volume separately both from app and physical buttons. Working OK.
Use case 3: 2 speakers, 1 room, stereo paired so it should become seen as a single speaker, potentially grouped with other speakers but irrelevant: control speaker volume stereo paired speaker set as one from app working OK but physical buttons still only adjust volume on a single channel. Definitely NOK.