There you go again, shooting down all my ideas. 🤯 Does the Google maps audio come through your Alpine or are you hearing it on the iPhone's speakers? If it is coming from the Alpine's speakers, do you get sound when bluetooth on the phone is switched off? Please forgive my curiosity, I haven't encountered a system such as yours before.
Thinking out loud: If you are getting sound in the car from Music, Maps, or any other source and the intermittent problem is limited to one or two apps, then logicalluy the issue would have to be with the apps. An issue with bluetooth, USB, or iOS would effect all apps equally. Apple provides the APIs and it is up to the application developer to communicate correctly via those APIs. Both Google Maps and Audible were developed for platforms other than iOS (or any other mac OS) and "ported" to iOS and Apple silicon which is not uncommon, but depending on how the port was made it has the potential for producing difficult issues to detect, troubleshoot, and fix. Apple's relentless 1 year development cycle does not make third party developers jobs an easier.
That said, my personal response would be to report the issue to Apple being careful to layout the situation in complete and painstaking detail including all everything we have discussed in this thread via
Apple Feedback and let Apple make the decision whether to forward the information to Google and/or Amazon. Apple might also identify something in iOS that is contributing to the issue. It really is worth doing.