this is closely related to this issue from dankMaterialShell, but also happens on noctalia and probably other shells: AvengeMedia/DankMaterialShell#333
It seems that the second a bluetooth device is connected, audio adjustments other than mute cease to function
When changing the volume through wpctl or something like pwvucontrol the shell (and actual volume) changes accordingly.
because I've identified this with both DMS and noctalia I've decided to open an issue here, but I'd need some help for getting any relevant logs and data for diagnosing and fixing the bug