This bug goes back to Poser 6 at least. I don't have Poser 5 installed any more, but I can say it isn't present in P4. I suspect the blame lies with the new preview code that was introduced along with the Firefly renderer in P5. For me, it's present whether I use OpenGL or SreeD preview modes.

What's surprising is that I haven't seen it before - although I have seen other strange preview effects.