Run the iOS native music player and play a song. Now press the volume buttons and change the volume of the song being played. The volume goes up and down, and that's it. Perfect.
Now open the Spotify app. Play a song. Click on the song album art to get into the menu for the song, showing album, artist, etc. Here is where you find the volume slider. Adjust the slider. What happens? The iOS volume icon overlay appears, which is a redundancy because the Spotify volume slider already shows this.
This is an interface design quality issue, and something that a company with the stature and quality of service that Spotify enjoys, ought to rectify.
The iOS white-on-black volume overlay display should only appear when the volume is adjusted and nothing else on the screen exists to indicate the volume level to the user. When the Spotify app already has a volume slider being adjusted, the overlay should be suppressed.
Please rectify this, since that redundancy is clearly a sign of bad interface design.