I have a straightforward test web form with a HiddenField and a TextBox. On submit, I update the HiddenField's value. On the target page, the TextBox value is received but HiddenField is blank.
TEST.ASPX
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="test.aspx.cs" Inherits="test" %>
<!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>Test</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:HiddenField ID="HiddenField1" runat="server" />
<br />
<asp:TextBox ...
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