Great question, Norbert. I had to reach out to our 3D rendering team. It is quite complicated, but the short answer is that the behavior is intended. I will do my best to answer why that is below.
MapView is a bit different than SceneView because MapView shows an infinite flat world map (with wrap around), whereas SceneView attempts to create an illusion of real-life perspective view (i.e. looking through a window). SceneView has a concept of "Field of View" (FOV), where 45 is considered an average non-distorted view. The allows changes in the window size to affect the FOV angle, which can lead to a very narrow "telescopic" view or very wide "fish-eye" view, which would be distorted. Because of this, we have slightly differing UX to account for these types of window size changes. You will see a similar UX in ArcGIS Pro where scale isn't guaranteed to stay the same as you change window size.
Hope that helps provide some insight as to why it behaves this way.
