ToolTip in An Infragistics WPF XamDoughnutChart

Question

I am using Infragistics XamDoughnutChart. For some reason the ToolTip text become strange. Here is the XAML code.

        <ig:XamDoughnutChart.Series>
            <ig:RingSeries x:Name="ringSeries"
                        ItemsSource="{Binding Samples}"
                        Brushes="{Binding SampleColors}"
                        OthersCategoryThreshold="0.0"
                        LabelMemberPath="Key"
                        ValueMemberPath="Value"
                        ToolTip="{} {Key}  {Value}%"
                        LabelsPosition="OutsideEnd"
                        LabelExtent="5"
                        RadiusFactor="0.8"
                        StartAngle="270"
                        Margin="5">
            </ig:RingSeries>
        </ig:XamDoughnutChart.Series>
    </ig:XamDoughnutChart>

I use string format "{} {Key} {Value}%" for the ToolTip, and it worked well. However, in the new version, it adds a prefix as shown.

Extra prefix of the ToolTip

Any idea on whether this is an Infragistics bug or there is a better format string?

The ToolTip should be something like "Auto Slices 43%".

If I change the format to "{} {} {Key} {Value}%", then another prefix string that is the name of the binding object will be shown, besides the "PieSliceDataContext". However, removing "{}" from the format as "{Key} {Value}%" wouldn't work at all.