I have been having this little (but quite annoying) issue for several weeks now. Whenever I change the volume by dragging the slider, it bounces randomly to other values than the one corresponding to the mouse position. This is way harder to explain than a good video so I created a gif (by the way, .mp4 file upload does not work here) :
Thank you for reporting this issue to us.
We just wanted to let you know that the right folks are looking into this.
We're not able to provide an exact timeline for a fix, but we'd recommend always keeping your Spotify app up to date to ensure you're on the latest version.