After iOS 11 update, whenever I listen to Spotify with my headphones and I get a call, the music continues to play. If I press pause in the mission control it doesn't stop either, so I have to force quit Spotify to make the music stop.
I found a post on Spotify forums about it from 2016, but I don't think it's relevant today(?). Either way there were no solutions to it.
I'm not entirely sure why it happens, but the quality of the music drops when I am on the call (it sounds like the music is picked up by the microphone from the person I am talking too). After I quit the call the quality resumes to what it used to be.
The problem does not exist when I use apple music.