I would like to provide an update regarding this matter. I have discovered that when using the cardo headset, playing the radio resumes smoothly after the GPS directions.
I have also come across an Android audio player called Player Pro, which enables the uninterrupted playback of music from a playlist in the background while the GPS provides directions. Background music volume is slightly lowered. Perhaps in a update, the developers could incorporate the feature of resume after transient bluetooth interruption to audio focus.

Agaun the app is called Player Pro for Android.