Problem with other apps that temporarily mute Spotify?

[Slackware]

Hi, 

when driving in my car, i have my HTC One M8 connected to my Car Audio via Bluetooth. 

I listen to spotify and also have a radar warner app running. 

This radar warning app mutes (or decreases) the sound of other apps for a short time to announce radars. 

With spotify tho, it seems as if spotify still is way louder than the other app. This causes me to not understand it. 

The sound settings of the other app are all maxed out. 

Is this a known bug? I have read about navigation apps having the same problem with Spotify. 

Can I fix this via some hidden settings? I remember it working like a year or half a year ago perfectly fine, but somehow now it does not work as intended. 

Any thoughts on this? 🙂