I've also recently experienced problems with TTS voice announcments.
My selected voice was English (UK), it then reverted to default English (American accent).
When I swapped to another language (French) there were noticable gaps over bluetooth where the French directions should have been (no voice directions).
When I swapped back to UK English then I got the American accent again.
There was a recent update to Google text-to-speech but I don't know if that is part of the problem.