A growing thread on the Android issue tracker is home to more and more reports of in-vehicle Bluetooth problems with the Nexus 5, 6, 5X, and 6P. The N5 and N6 allegedly only became affected when their Marshmallow updates rolled out, while the 5X and 6P, which run Marshmallow out of the box, have allegedly had problems from day one. Notably, the Bluetooth issue being cited supposedly was not present in the "M" developer preview releases for the Nexus 5 or 6, suggesting something changed recently to cause it.
The rather irksome bug manifests most commonly as follows. When attempting to dial out to a phone number when your phone is connected to your car via Bluetooth, the call will fail and the car's radio will resume playing whatever audio was on previously. Meanwhile, your phone will continue the call on the device, and though it will successfully connect, no audio can be heard by either party. Basically, it sounds like the phone audio Bluetooth connection is failing and the phone is then not properly reverting to device audio, though the fact that it is failing in the first place is obviously the larger problem. Incoming calls generally are reported to work, though some in the thread are claiming these are broken as well. Regardless, the safety benefits of the hands-free systems in vehicles are obviously compromised in such a situation.
Some posters are also reporting additional issues like random Bluetooth connection drops after a few minutes or problems properly loading contact names for given numbers. It's difficult to say if any of this is related or if they are completely different bugs with Bluetooth on Marshmallow, or if they are vehicle-specific. As to the culprit for the dial-out audio problem, Android 6.0 seems to be the common denominator here, and Google has assigned the bug with a "small" priority level. It's unclear just how widespread the issue is, but basically every major make of vehicle is represented on the issue tracker thread at this point.
We've reached out to Google for comment, and will update if we hear anything worthwhile back.
- Android Issue Tracker