I missed the part that you use IBL , but does not make any difference, it also will work using IBL setups in FF. actually if you set it up this way it works on all poser versions with any render setting and render engine . FF as well SF or Reality , a fast and easy way to set it up instead of messing around with all nodes ending up that it only works for one render version or Poser version. A one for all solution :)

Here a render in PP11 using IBL - IDL