Ok, I have some more information from today. This behaviour happens when for some reason you lose connection between the CarPlay device and the phone. Let me explain:
At some point my phone ran out of battery. I stopped, turned off the bike and therefore disconnected the CarPlay device. Put the phone under my seat where I have a charger. The phone never switched off, the CarPlay device did.
When I started the bike, and therefore disconnected the CarPlay device, the device reconnected to the phone and recalculated the route from my current position.
This is what happens: The route gets recalculated correctly. The Next Turn widget get updated once. The stats widget in the bottom left also updates once. After that moment, both widgets don’t get updated anymore, the (main) route screen however, continues to work correctly.
This issue got reproduced at the next stop where I had to take gas. CarPlay device switched off and later on again, phone reconnects, the two left widgets get updated once and never again.
I hope this helps finding the issue.