Bugs... The volume control on HU or steering wheel changes the currently playing audio. If navigation plays a message and you adjust the volume at the time, you would only affect the navigation control.
Not sure what you are saying. It seems that you are calling a Bug, the current, proper behavior of the volume control AFTER the update of Google Play Services. That is what we were hoping to achieve. After the update, adjusting volume during a Nav announcement affects only the Nav volume level, whereas an adjustment before or after a Nav announcement affects the source program volume level.
Prior to the update, the phone didn't release volume control back to the other audio input sources after a navigation announcement, and only the Navigation volume could be adjusted. That was the bug, IMHO.
BTW, not sure about Google maps, but in general you are not allowed to discuss any issues with beta testing outside of the beta forums.