Navigation ("Routing") is correct, but might be different for different Renderers.
Renderers often consider "surface=gravel" to be "unpaved".
Therefore navigation by "avoid unpaved roads" might be different.
Therefore (surface=gravel) is not preferred (IMO), unless it's really gravel.
Better: [surface=fine_gravel] + [smoothness=intermediate].
More correct is [smoothness=bad] (IMO), but again: Router might avoid both "surface=gravel" and "smoothness=bad".
To resolve: do not navigate by "avoid unpaved roads" if you want to navigate to these campsites.
Mapping as "highway=service" ('toegangsweg') or "highway=track" (unpaved or bad pavement) or "highway=unclassified" has no effect on navigation.