To reproduce: use the following code snippet and try to drag an item or click over a task item:

private ViewModel viewModel;

public Form1()
{
    InitializeComponent();
    viewModel = new ViewModel();
    viewModel.Init();
    viewModel.DateChanged += ViewModelOnDateChanged;

    this.radGanttView1.Ratio = 0.3F;
    this.radGanttView1.SplitterWidth = 7;
    this.radGanttView1.TitleMember = "Name";
    this.radGanttView1.ChildMember = "Id";
    this.radGanttView1.EndMember = "End";
    this.radGanttView1.ParentMember = "ParentId";
    this.radGanttView1.ProgressMember = "Progress";
    this.radGanttView1.StartMember = "Start";
    this.radGanttView1.TaskDataMember = "TimingRows";
    this.radGanttView1.LinkDataMember = "LinkRows";
    this.radGanttView1.LinkStartMember = "StartId";
    this.radGanttView1.LinkEndMember = "EndId";
    this.radGanttView1.LinkTypeMember = "LinkType";

    this.radGanttView1.DataSource = viewModel;

    this.radGanttView1.Click += radGanttView1_Click;
}

private void radGanttView1_Click(object sender, EventArgs e)
{ 
    this.radGanttView1.DataSource = null;
    this.radGanttView1.DataSource = viewModel;
}

private void radGanttView1_MouseEnter(object sender, EventArgs e)
{
    this.radGanttView1.DataSource = null;
    this.radGanttView1.DataSource = viewModel;
}

private void ViewModelOnDateChanged(object sender, EventArgs eventArgs)
{
    BeginInvoke(new Action(() =>
    {
        this.radGanttView1.DataSource = null;
        this.radGanttView1.DataSource = viewModel;
    }));
}

public class ViewModel
{
    public event EventHandler DateChanged;

    public BindingList<TimingRow> TimingRows { get; set; }

    public BindingList<LinkRow> LinkRows { get; set; }

    public ViewModel()
    {
        TimingRows = new BindingList<TimingRow>();
        LinkRows = new BindingList<LinkRow>();
    }

    public void Init()
    {
        AddRow(1, 0, "group", 3, 12);
        AddRow(2, 1, "Test1", 3, 4);
        AddRow(3, 1, "Test2", 7, 8);
        AddLink(2, 3, 1);
    }

    private void AddLink(int p1, int p2, int p3)
    {
        var link = new LinkRow(p1, p2, p3);
        LinkRows.Add(link);
    }

    private void AddRow(int id, int parentId, string name, int addDays, int howLong)
    {
        var row = new TimingRow
        {
            Start = DateTime.Today.AddDays(addDays),
            End = DateTime.Today.AddDays(addDays + howLong),
            Id = id,
            ParentId = parentId,
            Name = name
        };
        row.DateChanged += RowOnDateChanged;
        TimingRows.Add(row);
    }

    private void RowOnDateChanged(object sender, EventArgs eventArgs)
    {
        if (DateChanged != null)
            DateChanged(this, EventArgs.Empty);
    }
}

public class TimingRow
{
    public event EventHandler DateChanged;

    private DateTime start;

    public string Name { get; set; }

    public int Id { get; set; }

    public int ParentId { get; set; }

    public decimal Progress
    {
        get
        {
            return 0;
        }
    }

    public virtual DateTime Start
    {
        get
        {
            return start;
        }
        set
        {
            if (start == value)
                return;
            if (DateChanged != null)
                DateChanged(this, EventArgs.Empty);
            start = value;
        }
    }

    public virtual DateTime End { get; set; }
}

public class LinkRow
{
    public int StartId { get; set; }

    public int EndId { get; set; }

    public int LinkType { get; set; }

    public LinkRow(int startId, int endId, int linkType)
    {
        this.StartId = startId;
        this.EndId = endId;
        this.LinkType = linkType;
    }
}
---------------------------------------------------------------------------------
WORKAROUND:

private void radGanttView1_CreateDataItem(object sender, Telerik.WinControls.UI.CreateGanttDataItemEventArgs e)
{
    e.Item = new CustomGanttViewDataItem();
}

public class CustomGanttViewDataItem : GanttViewDataItem
{
    protected override void OnNotifyPropertyChanged(PropertyChangedEventArgs e)
    {
        if (this.GanttViewElement != null)
        {
            base.OnNotifyPropertyChanged(e);
        }
    }
}

public class CustomGanttView : RadGanttView
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadGanttView).FullName;
        }
    }

    protected override RadGanttViewElement CreateGanttViewElement()
    {
        return new CustomRadGanttViewElement();
    }
}

public class CustomRadGanttViewElement : RadGanttViewElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadGanttViewElement);
        }
    }

    protected override GanttViewGraphicalViewElement CreateGraphicalViewElement(RadGanttViewElement ganttView)
    {
        return new CustomGanttViewGraphicalViewElement(ganttView);
    }
}

public class CustomGanttViewGraphicalViewElement : GanttViewGraphicalViewElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(GanttViewGraphicalViewElement);
        }
    }

    public CustomGanttViewGraphicalViewElement(RadGanttViewElement ganttView) : base(ganttView)
    {
    }

    protected override IVirtualizedElementProvider<GanttViewDataItem> CreateElementProvider()
    {
        return new CustomGanttViewVirtualizedElementProvider(this);
    }
}

public class CustomGanttViewVirtualizedElementProvider : GanttViewVirtualizedElementProvider
{
    GanttViewBaseViewElement owner;

    public CustomGanttViewVirtualizedElementProvider(GanttViewBaseViewElement owner) : base(owner)
    {
        this.owner = owner;
    }

    public override IVirtualizedElement<GanttViewDataItem> CreateElement(GanttViewDataItem data, object context)
    {
        GanttViewBaseItemElement element = this.OnItemElementCreating(data);

        if (element != null)
        {
            return element;
        }

        if (owner is GanttViewTextViewElement)
        {
            return new GanttViewTextItemElement(owner as GanttViewTextViewElement);
        }
        else
        {
            if (data.Items.Count > 0)
            {
                return new GanttViewSummaryItemElement(this.owner as GanttViewGraphicalViewElement);
            }
            else if (data.Start == data.End)
            {
                return new GanttViewMilestoneItemElement(this.owner as GanttViewGraphicalViewElement);
            }

            return new CustomGanttViewTaskItemElement(this.owner as GanttViewGraphicalViewElement);
        }
    }
}

public class CustomGanttViewTaskItemElement : GanttViewTaskItemElement
{
    public CustomGanttViewTaskItemElement(GanttViewGraphicalViewElement owner) : base(owner)
    {
    }

    public override void Synchronize()
    {
        if (this.Data.GanttViewElement != null)
        {
            base.Synchronize();
        }
    }
}