Does it switch settings when you load a pz3/pzz ?

The preview settings usually will obey the scene that is loaded.

If your "preferred scene", the one you start with, is set to no hardware shading, then Poser should start with no hardware shading.

But if you open a scene that uses hardware shading, Poser will use hardware shading as long as the scene is open.

Unless you switch it off manually.

If Poser switches arbitrarily, then there must be a bug somewhere.