Workaround:
public Form1()
{
    InitializeComponent();
    this.radPivotGrid1.GroupDescriptorElementCreating += radPivotGrid1_GroupDescriptorElementCreating;
}

private void radPivotGrid1_GroupDescriptorElementCreating(object sender, Telerik.WinControls.UI.GroupDescriptorElementCreatingEventArgs e)
{
    e.GroupDescriptorElement.FilterPopup.PopupOpened -= FilterPopup_PopupOpened;
    e.GroupDescriptorElement.FilterPopup.PopupOpened += FilterPopup_PopupOpened;
}

private void FilterPopup_PopupOpened(object sender, EventArgs args)
{
    PivotGroupFilterPopup filterPopup = (PivotGroupFilterPopup)sender;
    RadTreeView tree = filterPopup.TreeViewMenuItem.TreeElement.TreeView;

    ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Value = 10;
    ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Value = 0;
}