Try changing the "Topography Style" for each subregion surface.
In the clip below the entire Topo Material = Grass
When I select the first subregion, change the Topgraphy Style = 3 and then Material = Pavers
Then I selected the second subregion and changed the Topgraphy Style = 4 and then Material = Asphalt, Pavement

This is in Revit 2022