New label appears at position of old label

[hpinkos]

1. Click the Destroy button to remove the first label
2. Click the Label 2 button to add a new label
3. Zoom in towards the middle of the globe. Eventually the new label will appear around the spot the old label was, even though the new label has no position

This is specific to labels, it does not happen with billboards. It also only happens when using the CLAMP_TO_GROUND height reference

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

var labelCollection = scene.primitives.add(new Cesium.LabelCollection({
    scene: scene
}));

var label = labelCollection.add({
    heightReference: Cesium.HeightReference.CLAMP_TO_GROUND,
    text: 'label 1',
    position:  new Cesium.Cartesian3(-748060.358557192, -4909764.442609902, 3988488.2144659692)
});

Sandcastle.addToolbarButton('Destroy', function() {
    labelCollection.remove(label);
});

Sandcastle.addToolbarButton('Label 2', function() {
    labelCollection.add({
        heightReference: Cesium.HeightReference.CLAMP_TO_GROUND,
        text: 'label 2'
    });
});

[mramato]

This sounds similar to a billboard bug I fixed in #5339. My guess is that label caching isn't clearing the clamped state.