The reason is the scene ready mechanism. Here is a (hacky) way to fix it:
Questionable animation | Babylon.js Playground (babylonjs.com)
The idea is this - if you load a model right after the scene is ready, the scene will not be ready and will not render. So loading the model on AFTER render will make it work as you (probably?) expect.
I am trying to think of a good solution for that. Assets container might be a good solution, or making sure you wait one frame before running the next load.