Not sure what to tell you. It's Samsung and all the changes they make to Android under the hood (the modes the phone goes into when USB is connected, how voice commands are handled, etc.). They break stuff when they don't follow Google's design standards and APIs. One of two things will have to happen: 1. Samsung will fix whatever they did in TouchWiz that broke compatibility (not likely), or 2. Google will attempt to update the Android Auto app to work around Samsung not following the standard they provided (more likely, but not guaranteed).
I doubt Pioneer needs to do anything (if they followed Google's instructions for how to interface with Android Auto).
As Phil likes to say, "there's a lot of moving parts". While that's true, it's also true that Google and Apple provided standards to the automobile and phone OEMs for how the radio head units are supposed to communicate with the phones (and vice-versa). Evidently, someone didn't follow instructions close enough.