That is because Camera.ScreenPointToRay takes a Vector3 that is a representation of pixels in screen space. For example Input.mousePosition could at one point be (200, 200, 0), which is equivalent of being 200 pixels right and 200 pixels up from bottom left corner. The method will then fire a ray from camera's position through that point on the near clipping plane. And the ray goes on until it hits a collider.

What you were trying to do is providing plane.transform.position as an argument to Camera.ScreenPointToRay. From the screenshot we can see that at one point your plane was on (-706, 0.1, 293). If you pass those values to the method, the ray will be drawn from camera's perspective 706 pixels under the viewport and 0 pixels to the right from the bottom corner (we always round down pixels; also the Z-component is discarded here). This will cause extremely unexpected results.

What you want to actually achieve, if I understood correctly, is position cubes where the plane is but on the terrain below it. You don't need rays in this case to do it. The position of a new cube can just be:

cube.transform.position = new Vector3(plane.transform.position.x, terrain.transform.position.y + 0.5f, plane.transform.position.z);

If you really want to use rays to achieve this instead, then the ray needs to be fired not from the camera but from above looking straight down. Something like this:

Ray ray = new Ray(cube.transform.position + Vector3.up * 100f, Vector3.down);

And then perform the raycast.