physic with SceneLoader.Append and PhysicsImpostor.HeightmapImpostor

Hello,

When I use an impostor BABYLON.PhysicsImpostor.HeightmapImpostor and SceneLoader.Append(), I get a bug. If I use another impostor, I do not have this bug and if I use SceneLoader.ImportMesh(), I do not see this bug.

Only with an impostor BABYLON.PhysicsImpostor.HeightmapImpostor  and SceneLoader.Append()

I made this PG test to show the bug. The terrain is not a ground of Babylon, but from 3ds max. this works with a land made with Babylon (CreateGround())

http://www.babylonjs-playground.com/#DPDV50#13