Hi there @JoshuaAM! Welcome to the Spotify Community.
Unfortunately, I'm pretty sure this is an issue with iOS and not the app itself. I haven't heard of another app that utilizes it's own volume among the rest of the OS. I do have to say that I'm not entirely a connossiuer of apps though so I could be wrong, but I have a pretty strong feeling about this.
Hey there you, Yeah, you! 😁 Welcome - we're glad you joined the Spotify Community! While you here, let's have a fun game…