Why?? Sonos has a maximum volume limit. If you’re going to auto turn down, why notify. If you can limit, why do either.
Side note: Sonos is atrocious when it comes to functionality and reliability, and it has only gotten worse recently. I can't discourage people enough from purchasing their product.
It seems very strange that the relative volumes would be so different on AirPods and via Sonos otherwise.
Your side note is totally true. Since the “new” app it has gone to hell. Has now joined the queue of things in need of replacements in this house.
It only makes sense as a problem if there is something about the unique combination of Sonos and music or it would have the same relative difference when played on Airpods.