I pinpointed the reason why the audio was cutting off for me - WAZE.
I’ll connect my iPhone, click the Waze app in CarPlay and then set a destination. While Waze is navigating and I’m cycling through audio sources (for me it’s: CarPlay, CD [yes I do have a cd inserted], AM, FM, SiriusXM, Bluetooth) that’s when the sound cuts off. It happens exactly after I attempt to cycle past whatever audio is playing on CarPlay. Be it iTunes, Spotify... From CarPlay to CD and then it bounces back to CarPlay - no sound. Turning the Waze app off solves it. Just open the app on your phone and then swipe it away.
All the other navigation apps, including the Subaru one, works fine. If you can live without Waze, you’re good. If not, pray that Waze or Subaru addresses it and comes up with an update. Don’t hold your breath.
Just thought I’d share. I hope this helps.