Volume reduction on iPhone notifications remains reduced

[Durchlaucht]

Plan

Premium

Country

Germany

Device

iPhone XR

Operating System

iOS 13.3.1

My Question or Issue

Problem reproduction:

1. I listen music with Spotify.
2. Suddenly a message comes in,

2a. the volume of the music gets reduced

2b. the notification dinnnng sounds can be heard. 
Expected result: the volume comes back
Actual result: the volume remains reduced until I go to app switcher and come back to Spotify

Support for my iPhone says, this is an app problem, no OS problem. 

I need a solution. 

Best, durchlaucht;)