Sample application is attached , which shows the BUG (Just change the file extension to RAR and extract X2)
 
When placing two nodes in a group, it appears that the last inserted node will always be considered as the front layer.
Try pressing the nodes and you'll see that each node is going to the front once you press it.
However if you press the front node in the par that overlap the back node you'll see that the back node goes to front.