QSceneLoader doesn't work if loading is done after the window is shown. The loader status is never updated and nothing shows up on the screen.
The problem is demonstrated with the attached application. Copy it to same level as other Qt3D examples, as it is a modified assimp-cpp example. The difference to original assimp-cpp is that the scene is loaded after a slight delay, so that it happens after the window is shown.
This is rather annoying, as it prevents any use case where the user wants to use scene loader to add entities to an existing scene.