This is a very small space you’re placing a movable object in, and the lighting changes very quickly. It’s not going to be accurate. If your skylight is static, it uses Lightmass. If your skylight is movable, it uses a reflection of the sky. If it’s stationary, it uses both.
Lightmass calculates not only the static indirect lighting for stationary objects, but also a full-blown lighting cache across the level and in the air for movable objects: the indirect lighting used on that object is the closest cache to the object. In order for the cache system to work properly, you need to place at least one lightmass importance volume: the caches will be placed within the volume. But it’s only accurate to the nearest 3 meters. On the top of the viewport, click View >Visualize > Volume Lighting Samples to see the indirect caches.
The skylight will also take a generic reflection from the sky and map it onto the reflection environment of all objects within its radius.
In one of your examples, it looks like it took the lighting cache when the lighting is at full intensity. At 0 intensity, it looks like it still calculated the reflection. Even materials that are completely black will still reflect light unless specularity is set to 0 in the material.