Volume ruler on iPhone in Spotify App keeps fluctuating after changing volume with spotify connect

[jhamw1]

Plan

Premium

Country

Netherlands

Device

(iPhone 8 plus) on Yamaha Reciever

Operating System

(iOS 12)

My Question or Issue

My volume ruler in the Spotify App on iPhone keeps fluctuating when changing volume with Spotify connect on my Yamaha Reciever. After I changed the volume on the iPhone, and release the ruler, it fluctuates to another level. When I connect to the Yamaha Reciever with Airplay, there is no issue. 

What I have tried:
- Reset router
- Reset iPhone
- Uninstalled Spotify app, and reinstalled on iPhone
- Cleared cache on iPhone
- Reset the Yamaha Reciever


nothing helps. Anybody has tips?