Create Composite Elements
The _makeComposite attribute is applied to a metaclass element, not a stereotype element. It defines whether an element is always made composite when created.
Unless you also use the _defaultDiagramType attribute to define the child diagram type, the child diagram created is the usual default diagram type for the metaclass.
A stereotyped package is not by default created with a child diagram, so you should use the _makeComposite attribute to ensure the child diagram is created.
The following example from BPMN shows that a BusinessProcess element is always created as a Composite element with a BPMN custom child diagram.