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) :
Hey there folks,
We're received your reports about the volume slider having issues in the desktop app.
Our tech teams are looking into this.
If you're experiencing this, make sure to try a clean reinstallation with the steps in this Spotify Answer and if the issue persists after that, add your +VOTE and details here:
Your device and its OS (operating system) version
The exact version of the Spotify app you're on
Does this occur when you have Offline Mode toggled on?
Any additional details for example if you're using Spotify Connect or a non-default audio interface when this happens, etc would be very helpful
We'll keep you posted here if we have any updates, thanks!