Stop my phone volume altering playback elsewhere

[Godwhacker]

So I just noticed that if I'm playing something on my Mac / PC, and I also have the Spotify app open on my iPhone, using the volume change buttons on the side of the phone actually alters the volume on the computer.

This is very impressive! But it's also not something I want, it's completely unasked for and there's no way to stop it happening.

If you're going to change the behaviour of my phone without asking you could at least give me some warning, and then allow me to reset it's behaviour. 

I've been a subscriber for several years now and I'm sick of crap like this. Sort it out please. 

[Peter__]

Does this only happen when Spotify is on the foreground on your phone?