I have experienced many infuriating permutations iOS these problems, throughout iOS 8-11, iPhones 6-X, and various brands of automobile entertainment systems.
Music plays automatically when Bluetooth audio is selected, even though I NEVER play music from my phone to the car. The music which plays is the first alphabetic selection from the available songs. Podcasts never resume automatically, unless it has only been a short time since the previous playback session. Occasionally, a podcast starts and plays for a few seconds, and then a music selection spontaneously begins to play instead.
In iOS 11 on iPhone X, a new problem symptom: (this usually while Waze is currently navigating to a destination) after letting the phone start playing the first available (alphabetic) song, I launch the Apple Podcasts app and select the podcast I wish to hear (no thanks to Apple, as there is no predictable way to effectively play podcasts in order of release, unplayed episodes only) - but the podcast audio is emitted from the iPhone’s internal speaker (probably related to the selection is Waze to output it’s audio to the speaker, otherwise it’s almost impossible to hear Waze unless there is also audio content playing on the car sound system).
And why would everyone always want to hear the first alphabetic song in their collection when they turn on their car?
I accidentally discovered a solution to the podcast audio mis-routing: swiping down the iOS Control Center immediately and consistently results in the audio switching to Bluetooth (assuming BT audio is already selection the car’s audio system) from internal iPhone speakers.
Because these problems have persisted for years through so many OS, Podcasts app and iPhone hardware versions, and because I’ve experienced similarly horrible results on vastly differing brands of car hardware, it’s apparent that no one really gives a crap about any of this actually working. It’s all shameful.