The ElementLeave event is called only when you move cursor from the element to the diagram area. If you move the cursor from one element to another this event is not called (but the ElementEnter on the new element is called correctly).
If someone is good enough at handling the mouse, he can move the cursor through a flowchart from the beginning to the end without causing a single ElementLeave event to be called by hovering the cursor over the elements and the connectors between them. Is this behaviour intended or not (bug or feature) ?