Our developer research it, then found the mainly reason of the point disappear is because the default snappint mode is TileSnappingMode.Snapping, which means when you zoom out to higher than level 1, the map always handle the tile image of level 1, when it's high enough, the shapes disappar, because it's too small.
So you can make it works just add this line:
layerOverlay.TileSnappingMode = TileSnappingMode.NoSnapping;
It makes the shape keep been drawn when you zoom out. The point will keep to draw follow its style, and the other shapes will be drawn only one pixel (generally you cannot watch it by eyes).
And now it have another problem now, when you keep zoom out, in certain level the point get changed the style, the solution for that is change the tile type in very high levels: layerOverlay.TileType = TileType.SingleTile
Wish that's helpful, and our developer will did more research to make sure whether it have other problem about it.