Distance culling behavior of cartographic polygon on tile

At a reasonable distance the polygon appears not to be working. It is added to a POlygon Raster Overlay with LayerKey=Clipping on a MaterialLayer instance with the standard clipping layer. As I get real close then the tile is clipped as expected.
This is true using the Cesium provided 3 layer material. It works as a permanent clip with the default material. I can’t find where this distance culling setting is so I can get it to cull at a distance. Can you help?