Hi
That’s because when using meshes (which the recast graph does), there is usually not a clear definition of “inside”, meshes are usually not completely closed, so the recast graph only cares about the actual triangles in the meshes.
You can use a RelevantGraphSurface component to mark the regions that you want to be included in the graph, but then you will have to add one manually to each tile that you want to keep. You can also use a RecastMeshObj component to mark the surfaces of some objects not generate any navmesh, but only act as blockers.