My Nexus 6P updated to 7.1 last night and the Pixel came today, so this likely is a 7.1 issue.
I have a 2012 GM Equinox that was working great with the Nexus 6P pre 7.1, Never tried it with 7.1 because it came after I was home and had no reason to be in the car and the new Pixel came today...