matakoglu, I have the same problem, I have a Note 4 with Android 5.1.1 too. Not only in bluetooth, but even not connected to anything (speaker sound) it doesn't duck or pause the music when a notification comes up.
I asked my friends about their android spotify experience, all of them are ducking (lowering volume when a gps instruction or notification comes up) or either pausing the sound for the notifications.
It seems to be related to Note 4.
I'm using Galaxy Note 4, with official (but rooted) Android 5.1.1, Spotify app version 4.10.868
-In attempt to solve this, I followed guides to in this community to clean reinstall the app. (Remove cache, data, app, looked if there were any folders left, rebooted and reinstalled from the direct link, repeated the process again using the google play link)
-Tested other GPS software and had the same result with all of them, no ducking or pausing, both audio streams play at the same time and volume making it impossible to understand gps instructions. (Waze, Google Maps and Sygic)
-Tested combinations of different settings in the app, all with no avail.
My other apps duck and pause with no problem in the same phone/system. I was using Rdio app until it went bankrupt and it ducked perfectly in every situation. The powerapp amp pauses the music for notifications and other music apps duck properly as well. Tested with Waze, Google Maps and Sygic.
This is paramount for me, I used Waze and Rdio almost 100% of the time I am driving. Now I'm on Spotify and I want to do the same, like my friends can. If there is anything I can provide to help developers to fix this bug I will gladly do it for them.
I hope some moderator can read this and acknowledge this problem.