Update Design-time UserControl Interface Based On Property Value?

I've created a UserControl with the following public property:

[Browsable(true)]
public string Text
{
get { return pnlLookupTable.GroupingText; }
set { pnlLookupTable.GroupingText = value; }
}

pnlLookupTable is, as you may have guessed, a Panel control. I can change the value of the Text property in the Properties window and it's reflected in the markup like it should be. However, the design view of the UserControl inside a page does not show updated GroupingText for the Panel. How can I get this to happen?

EDIT:

By request, here is the entire class to which that property belongs. You can see there's nothing special going on:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;

[Code]....

And here's the relevant part of the .ascx file:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="LookupTable.ascx.cs" Inherits="LookupTable" %>
<asp:Panel ID="pnlLookupTable" runat="server" GroupingText="Lookup Table">
<%-- Irrelevant content here. --%>
</asp:Panel>

I have changed some identifiers and other trivial things to protect the proprietary nature of the code.

I should also reiterate that I'm looking at a Web form in design mode that has my control added, and I'm changing the Text property. I want to see the change to the Panel's GroupingText visually in the designer.