Comment 23 for bug 774059

I can confirm that this is broken.
This part of the code seems to be responsible:
    switch (sScreen->priv->type)
    {
 case ScaleTypeGroup:
     if (spScreen->clientLeader != window->clientLeader () &&
  spScreen->clientLeader != window->id ())
  return false;

     break;

 case ScaleTypeOutput:
     if ((unsigned int) window->outputDevice () !=
  (unsigned int) screen->currentOutputDev ().id ())
  return false;

     break;

 default:
     break;
    }

As ScaleTypeOutput works and the code calling it is essentially the same like the one calling ScaleTypeGroup, I suspect the problem must be there...