I think Tesla needs a cache feature. Perhaps if you navigate to a destination with or without cell service maps should cache the whole route regardless. I thought this was common in most apps nowadays.
Are we all sure it doesn’t do that already? When I did my test, the car continued navigating the route for over 10 mins through a blind spot where it had no cell coverage (though the google satellite images gradually scrolled off the screen leaving only bare roads). This does suggest some amount of caching (which in any case all nav apps must do SOME of).
Perhaps the OP was referring to attempting to enter a route when he had no cell coverage?