Hmm, that's not really a solution. What I have done myself to work around the issue is to add a couple of text boxes on the bottom of the dominant group box, then join it using the normal arrow line. I have then changed the colour of the arrow line to be the same as the background. This makes the box stay for the most part in a sensible place, and I can drop successive boxes off the bottom. IT's a bodge, but does seem to work. I used a BPMN "Plain Start Event" to try and anchor the default state to the top of the chart, that seems to have worked...