I have updated the sample car/road 3D models to be glTF (as mentioned on https://forum.castle-engine.io/t/scene-load-in-documentation/ ). As a side-effect, this surprising behavior is now gone, because the glTF models don’t contain any Background.
So now simply adding something like Viewport.BackgroundColor := Yellow; to the simple source as in Loading, displaying a scene (simple loading of 3D models) | Manual | Castle Game Engine , https://castle-engine.io/code-samples/view_3d_model_basic.lpr would just work.
Of course the solutions mentioned by @kagamma still apply, if you decide to use X3D model with Background for any reason – you can edit Background node in X3D, or you can not make the model a MainScene (then Background node inside is ignored).
