Odd behaviour verified on mp 0.6. Looks like a bug. Here's a workaround
(only tested on 0.6).

Make the contained_by limited to x=0, not x=1. Then render the object w/o
the difference, as it's no longer required. So, your code should read:

 #declare Obj =
 isosurface {
  function {(Par(x,y,z) & -Par(x*1.25,y-0.2,z*1.25))}
  sign -1 method 2 max_gradient 2.648
  contained_by {box {<-1,0,-1>,<0,1,1>}}
 }

object {Obj
  scale 10 rotate y*10 translate <-3.25,3.35,20>
  pigment {rgb 1} finish {ambient 0 diffuse 1}
}

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