How to make the building of 3D tile in cesium be 3D？

I'm trying to put the geojson file into the 3d tile in cesium, but after the conversion, the building is not 3d but flat, I would like to ask if there is a problem in which part?

This is my code：

var tileset = viewer.scene.primitives.add(new Cesium.Cesium3DTileset({
    url: './tilesets/taichung_house/tileset.json'
}));
    
viewer.zoomTo(tileset, new Cesium.HeadingPitchRange(0, -0.5, 0));