Aha! My exact problem, too. I experimented a bit. I have ReSound Omnias, which are Apple MFi hearing aids. In Settings, Accessibility, Hearing Devices, Audio Routing, I have picked “Always Hearing Devices.” But CarPlay by DEFAULT apparently overrides that. If I just get in my vehicle and play a podcast, it will go to the truck speakers instead of my HAs. But in the podcast app or widget, I can touch the output icon, see what output options are available, and for streaming a podcast, change from my vehicle to my HAs, and that switch STICKS. It’s a different story if I try Apple Maps or Waze in CarPlay. The default option is the vehicle, again. If I try to switch to my HAs, the output quickly switches back to the vehicle. But if I go back to the Apple Podcast app, it still streams to my HAs! Perhaps some genius’s crazy reason for this for routing software is that the sound seems better if it’s coming from your vehicle touch screen (stereo sound centered in front of the vehicle) rather than somewhere between your ears, not connected to the map at all.
Today I called ReSound for tech support and I mentioned the odd CarPlay behavior with driving software. The ReSound tech was amazed with my audio routing settings that any sound went to the vehicle speakers at all. He thought the Always Hearing Devices setting should lock streamed output to my HAs. He asked what the source of the sound was and I said that I was pretty sure it was the phone as only apps that I had on my iPhone app appeared in CarPlay and either for Waze or Apple Maps, if I got out of my vehicle without shutting off routing, the routing app would still be running and giving me verbal instructions, now through my HAs after shutting off my vehicle. Since a bunch of folks in this thread report similar behavior, it seems CarPlay is at fault, not any particular brand of vehicle or HA. The tech I spoke to said he’d bring up the problem I mentioned with ReSound programmers and hoped they could induce Apple to make CarPlay more flexible (CarPlay is Apple’s program, not any HA OEM’s).