In Flat Earth (FE) theory, the Sun is not moving in a circle around the surface of the Earth. Instead, the Sun is thought to be a celestial body that is fixed in one stationary position near the North Pole. This means that when satellites or other objects move into the shadow of the Earth, they are blocked from direct sunlight and can suddenly disappear from view, as you experienced. This phenomenon is known as an eclipse, and it occurs due to the way the Sun's light interacts with the Earth's atmosphere, which is why the ISS will eventually reappear after a few minutes when it moves back into direct sunlight.