A tileground question?!

I create a TiledGround,and use submesh.isInFrustum(plane) to make each submesh visible if it is view of
camera.( var plane = new BABYLON.Frustum.GetPlanes(camera.getTransformationMatrix()))

you can see, in my scene,some submeshes on the left and top of ground will be hidden, but it wont happen to submesh on the right and bottom of the ground

I’m not sure what we are seeing in the picture? I assume the blue points are meshes, but how their visibility is related to the camera? As we are seeing them with the current camera, they are displayed?