Marshmallow was another great step forward for Android but like lots of new things, its not without its growing pains. There’s a growing thread on the Android Issue Tracker with reports coming in that Bluetooth is not functioning properly in Marshmallow regardless of the device. In particular, Nexus devices running Marshmallow are not playing well with in-car Bluetooth systems. Some users are having issues with the devices staying paired while others are having calls rerouted to the handsets as soon as calls are connected. In a world where distracted driving is an ever growing issue, this is a pretty big problem.
I’ve personally noticed the issue with my Nexus 6P. I initially thought it was just my car’s Bluetooth as I’d had some issues previously with other devices but after reading through some of the thread, my issues are identical to many other Marshmallow users out there. I use my built in Bluetooth system pretty regularly so hopefully this gets resolved sooner rather than later.