I connect my iPhone SE (iOS 10) to my car radio.
If I play songs from Spotify it works fine.
If I play local files which I synced from my Desktop-PC the bluetooth connection disconnects randomly and instantly connects again. Sometimes it doesn't reconnect and I even can't reconnect manually.
Due to this I first thought my radio is broken. But I exchanged it and the issue is still there. Since it always and only happens when I play local files it has to be an issue with Spotify or iOS.