Wokaround:

Private Sub GroupDescriptorElementCreating(sender As Object, e As Telerik.WinControls.UI.GroupDescriptorElementCreatingEventArgs)
    RemoveHandler e.GroupDescriptorElement.FilterPopup.TextBoxMenuItem.TextBox.TextChanged, AddressOf TextBoxMenuItemTextChanged
    AddHandler e.GroupDescriptorElement.FilterPopup.TextBoxMenuItem.TextBox.TextChanged, AddressOf TextBoxMenuItemTextChanged 
End Sub

Private Sub TextBoxMenuItemTextChanged(sender As Object, e As EventArgs)
    Dim tb As RadTextBox = TryCast(sender, RadTextBox)
    Dim popup As PivotGroupFilterPopup = TryCast(tb.Parent, PivotGroupFilterPopup)
    popup.TreeViewMenuItem.TreeElement.TreeView.EndUpdate()
End Sub