When using Android Auto, the Always-On Display remains active the entire time, which is unnecessary and drains battery. On Pixel and iPhone, the screen turns off appropriately to avoid heat and conserve power.
As a result, the phone runs noticeably hot, especially during navigation or music playback, even when not being touched.
It should automatically turn off the display during Android Auto under the 'auto' setting
This should be a matter of choice as I personally like it on. You can use Modes And Routines and create a routine to turn off when connected if this is how you prefer though