Keep it on driving/navigation mode during journey and it will not go offline. Lack of compass will not show direction at first, but when you start moving, it will start assessing directions.
Also keep traffic mode here, so it's forced to stay connected to show real time traffic.
One big drawback of Google Maps is, that if you take wrong turn or miss instructions, it re-routes to your destination using that wrong route. Instead, it should announce that that you took wrong route before re-routing. So it's best to keep it open in front of dashboard.
I usually keep it open during journeys on non familiar roads and it helps a lot, not just for navigation, but for safety as well. Letting one car go ahead of you also helps in absence of navigation.