Model ID not preserved upon copy()

Try this fiddle :

http://ift.tt/1koTu0o

It is working fine with 4.1 or 4.2. On 5.0.0.736 IDs differ. Am I missing something ?

Code:

Ext.define('TestModel', {
    extend: 'Ext.data.Model',
    idProperty: 'myId',
    fields: [
        {name: 'myId',     type: 'int'}
    ]
});

Ext.onReady(function() {
    var treestore = Ext.create('Ext.data.TreeStore',{
        model: 'TestModel',
        root: {
            myId: 1,
            expanded: true,
            children: []
        },
        proxy: {
            type: 'memory' ,
            reader: {
                type: 'json'
            }
        }
    }) ;
    
    var oldId = treestore.getRootNode().getId(),
        newId = treestore.getRootNode().copy(null,true).getId() ;
    if( oldId != newId ) {
        Ext.MessageBox.alert('Failure','IDs differ !') ;
    } else {
        Ext.MessageBox.alert('Success','IDs are identical') ;
    }
}) ;