Undo removes styleName attribute

Is the the bug in my test case or in the TLF code (Flex 4.0.0.13555)?

public class UndoBugTest extends TestCase
{
    //==========================================================================
    //
    //  set up and tear down
    //
    //--------------------------------------------------------------------------
    private var app:Application
    private var container:UIComponent
    private const ANY_URL:String = "http://livedocs.adobe.com/"
        
    protected function get initialImport():XML {
        return <TextFlow color="#000000" fontFamily="Tahoma" fontSize="14" fontStyle="normal" fontWeight="normal" lineHeight="130%" textDecoration="none" whiteSpaceCollapse="preserve" xmlns="http://ns.adobe.com/textLayout/2008">
                              <p>
                                <span>aaa</span>
                              </p>
                              <p styleName="h1">
                                <span>bbb</span>
                              </p>
                            </TextFlow>
    } 
    
    protected function get initialImportString():String {
        return null
    }
    
    protected var initialImportXMLString:String = initialImport.normalize().toXMLString()
    
    protected var editManager:IEditManager
    protected var undoManager:IUndoManager
    protected var textFlow:TextFlow 
    
    
    override public function setUp(): void
    {
        container = new UIComponent()
        var controllerOne:ContainerController = new ContainerController(container, 500, 500)
        app = Application(FlexGlobals.topLevelApplication)
        app.addElement(container)
        
        textFlow = TextConverter.importToFlow(initialImport, TextConverter.TEXT_LAYOUT_FORMAT)
        
        undoManager = new UndoManager()
        editManager = new EditManager(undoManager)
        textFlow.interactionManager = editManager
        
        textFlow.flowComposer.addController(controllerOne);
        textFlow.flowComposer.updateAllControllers();
    }
    
    override public function tearDown():void
    {
        app.removeElement(container)
    }
    
    //==========================================================================
    //
    //  tests
    //
    //--------------------------------------------------------------------------    
    public function test_crossing_paragraphs(): void
    {
        editManager.selectRange(1,5)
        editManager.applyLink(ANY_URL, "_self", true)
        
        undoManager.undo()
        assertEquals(initialImport, exportXMLString())
    }
     protected function exportXMLString():String {
        var export:XML = TextConverter.export(textFlow, TextConverter.TEXT_LAYOUT_FORMAT, ConversionType.XML_TYPE) as XML
        return export.normalize().toXMLString()
    }
}