Oh, the issue is that the key "loading" is already used for the game loading bar of the game.
As the loading bar and splash screen are loaded before everything in the game, including the yaml files, they can't be specified there. The game loading bar and splash screen paths are in the config file of the game, and the actual loading of them are in the bootstrap file for RenJS.
If your loading bar has nothing to do with that, and it's just another kind of loading bar for the gui, you will just have to change the key name, for example use loading2, and you will then have that key available to use anywhere from the gui.