This seems like some sort of bug to me after running
your code. If you add some other object to the scene
it starts behaving normally. Adding...
sphere {<0,-10000,0>,1 pigment{White}}
and increasing the emission to 0.05 looks OK.

It has something to do with the way you made the
object as well, since...

#declare Disk2 = difference {
   object{Cube}
   object{top}
   object{bottom}
};
object{ Disk2 //Disque
etc

Also looks ok, as the only object in the scene.
Somehow when a ray is cast at Disque it isn't
finding any other objects so it looks different,
probably exits early out of some calculation.

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