This sort of thing happens with other programs that are used to make morphs.
A couple of standard solutions are as follows :-

Try using the .OBJ file from the RuntimeGeometries folder.
This is always best as I believe that when loaded via a figure file, there is a small position change.

The other thing that may be happening is a loss of precision in the coordinates during the round trip, the cure for which is to use ObjAction Scaler (from the FreeStuff) to scale up before import and scale down before importing back into Poser.