You have to interrupt the phloem, which is where the sugars go down from leaves to roots. But, the cambium can become either xylem or phloem, depending on the needs of the tree.
So if you leave cambium, the phloem will regenerate. If you go to deep, and interrupt all the sapwood, the leaves will not recieve sap flow and die , which seems like your case. Sometimes, the leaves just receive enough sap to live, but it looks like autumn above the layer (I had a maple going all red, but leaves did not fall off and the branch got successfully layered).