This feature has been broken for me since updating to iOS 11. 11.1 does not fix it. I use an iPhone 6s on T-Mobile.
I drive a 2014 BMW 320 and I have this problem when playing audio over bluetooth or over USB. Sometimes spoken nav doesn't work at all over bluetooth—the audio just pauses where a direction would be given and then resumes without giving directions at all. My wife has a CarPlay receiver in her car and everything works fine in that scenario. I recognize the bluetooth implementation in my car might be wonky, but this feature did previously work consistently when I had the phone plugged in over USB.
I typically use Overcast for playing podcasts, but yesterday I tried the same podcast in Overcast, Castro, and Apple Podcasts. Plugged into USB, using the two third party apps, the audio pauses briefly and the nav directions start speaking, then the (podcast) audio resumes while directions continue speaking over them, making it hard to listen to either. In my brief test, Apple Podcasts (the app) worked fine—it paused when spoken directions were given, and remained paused during the full duration of the directions.
I'm not switching podcast apps just for this... Has there been an API change in iOS 11 that podcast app developers need to implement? I haven't heard about this bug anywhere else. It's super frustrating...