I had this on a recent project. It was V4's hip that disappeared. When I examined the PZ3 file with an editor, it turned out that the:

{
storageOffset 0 0 0
geomHandlerGeom 13 hip
}

lines had disappeared from the hip actor. I edited them back in and all was well again, however that may not be helpful if you aren't happy editing Poser files. Of course this may not be the cause of your problem anyway...

I have no idea what caused it in my case, except that I had been doing a lot of work on this scene, trying various things, and that often results in what I like to call "Poser Rot". Some scenes have got to the point where I have no option but to recreate them from scratch if I want to continue working with them.

Forewarned is forearmed, as they say, so save early and save often - preferably to sequentially numbered files rather than overwriting the same one every time.