Google's Pixel smartphones have a lot to love about them, from their awesome camera to their Android 7.1 Nougat software, but with all that there is to love there's also bound to be some stuff that users won't love, and that seems to have manifested in the form of Bluetooth pairing problems. Specifically, it would seem that some users are having issues with Bluetooth connections between their vehicle and the Pixel devices when it comes to pairing the phone to the car, as well as staying connected after pairing.
It isn't immediately clear how wide spread this problem may be, and there doesn't seem to be any details as to what's causing the issue in the first place, leading some to believe that it could be the Pixel and Pixel XL but that it could also have something to do with the Android 7.1 Nougat software in general. For now, there is no way to know and likely won't be until Google opens up about the problem and begins to investigate the issue followed by a fix. The good news is that Google already looks to be traveling down that path as they've been responding to users with this issue in the Pixel Community. According to the details of the user reports, some users are able to connect their devices for streaming music but the Bluetooth will drop out after a call comes in and disconnects the device.
As for speculation that it has something to do with the Android 7.1 Nougat software, the issue is said to be happening on more than just Pixel and Pixel XL devices, as some users have mentioned the same problems with Android 7.1 on their Nexus phones. Both the Nexus 5X and the Nexus 6P are compatible with the Android 7.1 Nougat developer preview that went out earlier this month, although it doesn't appear to be mentioned which device is having the Bluetooth connectivity issues, if it's not actually both. If you're having trouble with connecting your device to your car, then at the moment the only thing that's possible to do is to wait for Google to figure out what the root cause of these connectivity issues are.