I have seen that there is a recurring problem with S21 not interfacing well with Android Auto. Apparently the problem is with Google. I have other Samsung Galaxy phones and they work flawlessly. Not sure if Samsung is pressing Google for a fix to the problem. Android Auto will start up once you connect the USB to the phone and the head unit, and then run for about 1-5 minutes before stopping. I searched Android Auto user boards and saw this was a common problem raised by others.
kindly check for newer software updates on Play Store
and have this reported via Members app
With my S8, the Maps hang regularly. Now with S21 Ultra, it cannot even load the route and the Maps crashed. Horrible! Is Samsung doing anything?