Quick answer
To clear fog, collect the map fragment for that section from Ruk. Walking through the area is not enough. Use the crow/Ruk markers as your search target; the fog disappears when you take the fragment from the pedestal.
The mechanic in plain language
The fog is not an exploration-percentage meter. Mortal Shell II treats the map like a collectible system: each reveal comes from a fragment.
That means you can stand inside a fogged region and still need to find its Ruk location.
What to do when a region will not reveal
- Open the map and look for any remaining crow/Ruk marker.
- Fast travel to the closest activated Beacon.
- Follow the route to the statue/pedestal.
- Interact with the map item.
- Reopen the world map and confirm that section has cleared.
If you are trying to clear everything, continue with the 11 Map Fragments guide.