While navigating tunnels GPS signal is often lost and route (wrongly: Waze often places the vehicle position on a nearby street) recalculated, or worst the position updating stops completely. The former case is a bit annoying but the latter is worst because you can easily lose the exit if it’s right after the tunnel end, since Waze recalculate the correct position only when the tunnel is over and the GPS signal restored.
Despite the GPS positioning difficulty, I noticed that very often the speed indication is still right.
By delimiting on the map the tunnels beginning and end Waze would know when the vehicle is entering a tunnel.
When GPS signal is lost or week: I would suggest to lock the rerouting option when inside a tunnel, unless the GPS signal is fully restored, and use the detected speed (if available), the last available position and the tunnel length to estimate the position of the vehicle.
This way it would stop to reposition the vehicle on a nearby street and rerouting when inside a tunnel, and if speed is available it would be able to estimate the position.