The new update now has the Assistant UI only cover the very bottom of the navigation where the ETA is shown. Asking it to do things like call or text people now keeps you in the navigation app rather than pulling you out of it. This is incredibly useful as the phone leaving the nav screen has been frustrating to say the least (and is a potential safety risk).
The feature seems to be rolling out to everyone. Give it a shot by hitting the mic or saying “Hey Google” while navigating to see if you have the new unobtrusive UI!