Workaround: create a custom Top10FilterOptionsDialog

public Form1()
{
    InitializeComponent();

    this.radPivotFieldList1.DialogsFactory = new MyDialogsFactory();
}

public class MyDialogsFactory : PivotGridDialogsFactory
{
    public override ITop10FilterOptionsDialog CreateTop10FilterOptionsDialog()
    {
        return new MyTop10FilterOptionsDialog();
    }
}

public class MyTop10FilterOptionsDialog : Top10FilterOptionsDialog
{
    public override void LoadSettings(Telerik.Pivot.Core.Filtering.ITopGroupsFilter filter, string fieldName, IEnumerable<string> aggregateNames)
    {
        base.LoadSettings(filter, fieldName, aggregateNames);

        if (filter != null)
        {
            ((RadDropDownList)this.Controls["groupBox"].Controls["dropDownTopBottom"]).SelectedValue = filter.Selection;
        }
    }
}