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.