If this effect is causing you problems in a real scene, here's really
ugly workround that will get you going:

#declare my_isosurface = isosurface { ...

object {my_isosurface no_shadow}
object {my_isosurface scale 0.999}

The scale value plus the accuracy value should add up to 1.0 or less, so
scale 0.999 works with your accuracy value of 0.001. If you want more
accurate shadows you can use accuracy 0.00001 and scale 0.99999

Obviously, in your test scene there's no need for the shadow casting
copy, because the shadow isn't case on anything other than itself, and
it's those self shadows that are the problem.

-- 
Mike Williams
Gentleman of Leisure

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