Polyline volume with Callback Properties creates disfigured shape

Setting up a polyline volume with the positions and shape properties set to callback properties causes the volume to become disfigured. When I switch to using a static variable instead of the callback property the shape looks like it was intended to, in this case a triangular prism. Here are two bits of code you can plug into the sandcastle to take a look at what I'm talking about.
Without callback property (Works as intended)
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var viewer = new Cesium.Viewer('cesiumContainer');

var shape = [];
shape.push(new Cesium.Cartesian2(0,0));
shape.push(new Cesium.Cartesian2(-30,-18));
shape.push(new Cesium.Cartesian2(30,-18));

var polylinePos = [];
polylinePos.push(new Cesium.Cartesian3(-2464387.2699629106, -4781768.957526949, 3415187.6152766007));
polylinePos.push(new Cesium.Cartesian3(-2464386.0410873345, -4781766.231500047, 3415192.2873785086));
polylinePos.push(new Cesium.Cartesian3(-2464384.974105165, -4781763.478022626, 3415196.881609544));
polylinePos.push(new Cesium.Cartesian3(-2464383.674528509, -4781760.956392176, 3415201.3201015787));
polylinePos.push(new Cesium.Cartesian3(-2464382.213054812, -4781758.462205194, 3415205.8364595124));
polylinePos.push(new Cesium.Cartesian3(-2464380.7363797217, -4781755.938521874, 3415210.4047271665));
polylinePos.push(new Cesium.Cartesian3(-2464379.19890381, -4781753.296863412, 3415215.180640723));
polylinePos.push(new Cesium.Cartesian3(-2464378.0156161455, -4781750.659294037, 3415219.6969912923));
polylinePos.push(new Cesium.Cartesian3(-2464376.8323265836, -4781748.021721327, 3415224.21333946));
polylinePos.push(new Cesium.Cartesian3(-2464375.4643401965, -4781745.36735203, 3415228.885421247));
polylinePos.push(new Cesium.Cartesian3(-2464373.9572566673, -4781742.784664825, 3415233.5575004644));

var polylineVolume =
viewer.entities.add({
polylineVolume: {
show: true,
positions: polylinePos,
shape: shape,
material: new Cesium.ColorMaterialProperty((Cesium.Color.YELLOW.withAlpha(0.3))),
outline: true
}
});


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With Callback Property (volume becomes disfigured)

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

var shape = [];
shape.push(new Cesium.Cartesian2(0,0));
shape.push(new Cesium.Cartesian2(-30,-18));
shape.push(new Cesium.Cartesian2(30,-18));

var polylinePos = [];
polylinePos.push(new Cesium.Cartesian3(-2464387.2699629106, -4781768.957526949, 3415187.6152766007));
polylinePos.push(new Cesium.Cartesian3(-2464386.0410873345, -4781766.231500047, 3415192.2873785086));
polylinePos.push(new Cesium.Cartesian3(-2464384.974105165, -4781763.478022626, 3415196.881609544));
polylinePos.push(new Cesium.Cartesian3(-2464383.674528509, -4781760.956392176, 3415201.3201015787));
polylinePos.push(new Cesium.Cartesian3(-2464382.213054812, -4781758.462205194, 3415205.8364595124));
polylinePos.push(new Cesium.Cartesian3(-2464380.7363797217, -4781755.938521874, 3415210.4047271665));
polylinePos.push(new Cesium.Cartesian3(-2464379.19890381, -4781753.296863412, 3415215.180640723));
polylinePos.push(new Cesium.Cartesian3(-2464378.0156161455, -4781750.659294037, 3415219.6969912923));
polylinePos.push(new Cesium.Cartesian3(-2464376.8323265836, -4781748.021721327, 3415224.21333946));
polylinePos.push(new Cesium.Cartesian3(-2464375.4643401965, -4781745.36735203, 3415228.885421247));
polylinePos.push(new Cesium.Cartesian3(-2464373.9572566673, -4781742.784664825, 3415233.5575004644));

var polylineVolume =
viewer.entities.add({
polylineVolume: {
show: true,
positions: new Cesium.CallbackProperty(function(time, result) {
return polylinePos;
}, false),
shape: new Cesium.CallbackProperty(function(time, result) {
return shape;
}, false),
material: new Cesium.ColorMaterialProperty((Cesium.Color.YELLOW.withAlpha(0.3))),
outline: true
}
});


Reproduce the issue in Cesium Sandcastle using the two provided polyline volume examples, then trace how CallbackProperty values are handled for positions and shape. Compare the callback-generated volume with the static version; done means both produce the intended triangular prism without disfigurement.

Written by the indexing model from the issue text.