Bug setting label position and heightReference

[hpinkos]

In the following example, the Change Position button stops working after you click the Unclamp button. It resumes working if you hit the Clamp button to reset heightReference: Cesium.HeightReference.CLAMP_TO_GROUND;

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var cesiumTerrainProviderMeshes = new Cesium.CesiumTerrainProvider({
    url : 'https://assets.agi.com/stk-terrain/v1/tilesets/world/tiles',
    requestWaterMask : true,
    requestVertexNormals : true
});
viewer.terrainProvider = cesiumTerrainProviderMeshes;

var labels = scene.primitives.add(new Cesium.LabelCollection({
    scene: scene
}));
var label = labels.add({
    text: 'Test label',
    heightReference: Cesium.HeightReference.CLAMP_TO_GROUND,
    position: Cesium.Cartesian3.fromDegrees(-75.1641667, Cesium.Math.randomBetween(0, 50), 150)
});

Sandcastle.addDefaultToolbarButton('Clamp', function() {
    label.heightReference = Cesium.HeightReference.CLAMP_TO_GROUND;
});

Sandcastle.addToolbarButton('Unclamp', function() {   
    label.heightReference = Cesium.HeightReference.NONE;
});

Sandcastle.addToolbarButton('Change Position', function() {   
    label.position = Cesium.Cartesian3.fromDegrees(-75.1641667, Cesium.Math.randomBetween(0, 50), 150);
});