I'm not sure this is bug or specfication of Poser,
but Poser can not make correct shadow map when the object is small.

The work around we found are
(1) Use raytraced shadow
or
(2) Load more bigger object in the scene (or more objects outside the camera)
or
(3) do not cast shadow

Caution: following link is japanese.
http://www.forum3d.net/bbs/index.php?topic=835.0