This is probably a bug. Please file a JIRA issue for it.
Okay so this is worse than I thought. It's not just the transition text that's snapped back into place. It's everything. A lot of my nodes are jumbled up and placed at random locations. So there is this web of transition lines running everywhere to reach the nodes. You can't see what's going on. Each time I open my process definition in the GPD to work with it, I have to spend 10 to 15 minutes re-aligning everything. Is this common? Does anyone else have the same problem? There are two of us in my office with this same problem, although we are using exactly the same environment etc.
I have updated the JIRA issue mentioned above, and included a screenshot.
Encountered the same issue when playing around with version 3.1.0.
As a temporary fix, you can put the end-state node as last node in the gpd.xml.
It seems like when this node is not the last one in the file, the node placement gets mixed up.
Weird, mine works the other way round. If the end node is the last one in the gpd.xml, it mixes up the diagram. I have updated the JIRA issue to reproduce this problem.