I found out that you have to start Poser fresh and repeat the steps to see this behaviour. Sometimes the problem disappears if you change rendering mode, but not always in all scenes. It seems the problems worsens with certain props. In my case with the ground plane in Lore-Naz scene from Daz I cannot get it at all to work. Those shadows remain from a standing figure - even if you load a posed figure which you have saved before. This only happens in firefly