Odd problem. Latest MRA 4.1.3-311 on iPhone 15 running iOS 17.5.1.
When connected to my Cardo the instructions work just fine along with music and mesh intercoms.
I bought a 5" Carplay box from Ricoel. When it turns on I have the bluetooth setting to run all audio through CarPlay rather than on the CarPlay device. It works great with Spotify and comms, but the turn-by-turn from MRA is gone. I tried Google Maps/Waze/Scenic and they all work fine. If I turn off the CarPlay unit the audio instructions from MRA nav start coming through the headset again.
I tried a few different voices to no avail. Seems like a bug to me.