NavMeshSurface on Generated Mesh Won't Bake

I have a 2D Tilemap that I'm using to make separate areas for my navmesh. The sidewalks are the best place to walk, the streets are the worst, etc...
I will create a mesh for each different area of the navmesh in turn using C# and iterating over this Tilemap.

The white mesn in the picture below is the generated mesh I'm working with.

If I bake the NavMeshSurface the baking is over quickly and nothing is walkable. The NavMeshSurface does not turn blue to indicate it is navigable.

If I take a completely separate plane, add a NavMeshSurface, and bake it will bake both the plane and the NavMeshSurface on the "WalkableSurface" GameObject.

You can see the square plane in the middle of the picture below.

Why does the WalkableMesh object not bake without this plane?

Why does the plane bake the WalkableMesh?