It seems that setting a new background url for a scene where that url hasn’t been loaded yet, doesn’t make it appear. Reloading works, but it doesn’t work on the moment. Only seems to happen with The Forge, and could be related to how the URL is changed because the PIXI loader resources don’t end up having the url in it, but the thumbnail generation still works.
With the new CDN and no more hack of the PIXI loader, this is fixed.