I figured out a fix!!!
Go to Settings>Sounds and Vibrations>Advanced Sound Settings>Seperate App Sound
Turn the switch to the "On" position
Click on "App" and choose your messenger app and any other app you want to be heard through the phone.
Backup to the previous screen and press "Audio Device"
You should be all set. You will now play your message/ phone sounds through your phone without losing audio through your Bluetooth device!
For some reason, it stopped working. Back to square one. I'm sorry that I posted this as a solution!
MAYBE, JUST MAYBE, this will help, when you have your phone connected to your vehicle, go to bluetooth settings, and click the settings icon by the name of your vehicle, and click message access.
When I'm driving the vehicle's Bluetooth is active and am able to hear my notifcation through the phone but not through my headset if it's powered up which I need for my work. Samsung.....get your act together or else you'll lose some customers