I see what you mean. But i cannot replicate it.
I am also on "Offline" navigation. But i can select my route from my online collection. That is one big difference. Even with WIFI of i can select a route from my online collectiom
When i selected a route from my "downloads" folder (for offline navigation) i can still navigate.
I also did a test where i disabled WIFI and Mobile data. In this case i was, just like you, diverted to the downloads folder. Here i select a route. Pressed on Navigate and the route did start on my phone