Required Validation Message Disappear after partial Postback

I have 3 controls inside Update Panel Textbox with ASP.NET Required Validation DropDownList do postback Submit Button My Problem Happened when I press on Submit Button and The Required Validation Message appear and then select anything from Autopostback DropDown which make the validation Message Disappear ... Any Suggestions or workaround to overcome that ??
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication5.WebForm1" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head runat="server"> <title></title> </head> <body> <form id="form1" runat="server"> <div> ...