Strange. I cannot confirm the problem if I copy the sphere_sweep code 
into my own scene. I shows with /your/ scene code.

#version 3.7;

global_settings {
   assumed_gamma 1.0
}

camera {
   location  <0.0, 1.5, -4.0>
   direction 1.5*z
   right     x*image_width/image_height
   look_at   <0.0, 0.0,  0.0>
}

sky_sphere {
   pigment {
     gradient y
     color_map {
       [0.0 rgb <0.6,0.7,1.0>]
       [0.7 rgb <0.0,0.1,0.8>]
     }
   }
}

light_source {
   <0, 0, 0>            // light's position (translated below)
   color rgb <1, 1, 1>  // light's color
   translate <-30, 30, -30>
}

#declare R = 0.07;

sphere_sweep
   { cubic_spline 6,
     <1, -1>, R,
     <-1, -1>, R,
     <-1, 1>, R,
     <1, 1>, R,
     <1, -1>, R,
     <-1, -1>, R
     bounded_by { box { -<1.5, 1.5, R>, <1.5, 1.5, R> } }
     pigment { red 1 }
   }

Thomas

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