Ahh yes this is an unfortunate quirk of the cubemap system.
Please add the following line after:
    cubeMapAsset.ready(onAssetReady);
    cubeMapAsset.loadFaces = true;        // set loadFaces flag, ugh
    self.app.assets.load(cubeMapAsset);
If you don’t set loadFaces flag on the asset it will refrain from loading them.