From Chuck directly:
The zoom in navigation is set to one of two zoom levels based on your speed and distance to the next direction. If you change the zoom when this is set, then it will stay at that zoom level until you exit navigation and come back. If you want to disable dynamic zooming, change the option in navigation settings for Default Zoom to a specific zoom level from Dynamic and set the option for setting the zoom when nearing a turn never.
Hope that helps.