Unable to receive audible indication of text message when using Bluetooth in vehicle

That's because the audio from your phone is being routed to the head unit which supports A2DP audio streaming. If you're not listening to the bluetooth audio source on the stereo, you won't hear the notifications.

Realistically, you should be using Do Not Disturb while Driving to silence such alerts anyway so you don't kill someone trying to check your text messages when you should be paying attention to the road.

Have you enabled Do Not Disturb While Driving?