Why does the bottom half of the image show a shadow like the horizon when the background type is None and there is no object?

In order to apply Post Processing to only some of the objects, I thought of separating the camera and the layer. Camera 1 draws only the objects to which Post Processing is applied, and Camera 2 draws the background. In this case, I set camera 2 to be drawn first. The part of the background drawn by Camera 2 that overlaps the object captured by Camera 1 was drawn correctly, but the part of the background where nothing was captured by Camera 1 had a shadow like a horizon line. Is there any way to get rid of this shadow-like horizon?

Unity 2020.3.10ｆ1 Unity HDRP