This is strange.
While it does not solve your problem, try this slightly varied code:

camera{location <10,10,-10> look_at <0,0,0>}
background{rgb 0}

light_source{<10,10,-10>, rgb 6
}

isosurface {
    function {sqrt((x*y)^2+(y*z)^2+(z*x)^2-3)}
    max_gradient 36
    accuracy 0.00001
    contained_by { sphere {<0,0,0>,6} }
    pigment {
      rgb <1,0,0>
     }
}

double_illuminate fixes is to some avail, but gives strange results anyway.
Perhaps you should report this to p.b-t? Higher max_gradient or lower
accuracy did not help at all.

Marc-Hendrik

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