Constrained groups do not works against props. You can make it choreographed.

Maybe it is a bug, but if there is any magnet different Poser version can select different way child props. Default setting should not include magnets o colliding group, but maybe P9 does not recognize it as a magnet.

Nevertheless I think that transparent prop resolve problem faster and easier then SM fix Poser. Moreover you can join hair strands and head phantom as a 1 prop (choreographed group)

sample