I am experiencing something strange:

When playing Spotify from my iPhone 7 on my wireless B&W Zeppelin speaker, the volume is jumping up and down. Sometimes Spotify just turns up the volume to max, which is unbearable to listen to. I'm afraid the speaker will blow at some point.

I can sit with my phone and watch the volume slider suddenly just sliding to the right all by itself.

There is nothing pressing on the phones volume buttons. It is clean and a new phone. When going out of the app i can turn the call volume up and down, no problems with the button doing anything by itself anywhere else than in Spotify.

My wife has an iPhone 6 and she has no problems.

What is going on here?

See the attached video of the issue.