This worked for me! At least for the time being anyway.
I did not have Vocal Shortcuts enabled, but set a fake one up, then turned it off, deleted the shortcut and turned my iPhone 13 Pro iOS 18.0.1 off and on again (not sure if that contributed or not, but might as well be thorough!) and had my partner send me a text after I started a playlist in Apple Music, and the volume ducked low while Siri read it aloud in my AirPods Pro 2, and then the volume went back up to normal, as expected!
I'll update this if the problem returns, but I haven't seen a working solution yet, so figured I'd leave this here and maybe it can help someone else.