C# - FormsAuthentication AuthCookie Is Null Only For Some Users?

I am experiencing a strange problem with asp.net forms authentication. This problem only occurs for 3 users out of 30+ users that have successfully logged in. I am using very basic auth code that I have used many times and have never seen this problem. After the users successfully authenticates and the auth cookie is created, cookie added, and response.redirect to FormsAuthentication.GetRedirect(userid, false) is called. The Application_AuthenticateRequest method in Global.asax is hit.

// Extract the forms authentication cookie
tring cookieName = FormsAuthentication.FormsCookieName;
HttpCookie authCookie = Context.Request.Cookies[cookieName];
if (null == authCookie)
{
// There is no authentication cookie.
return;
}

So immediately after a "good" cookie is saved and the redirect occurs the cookie is null. I have run the code through the debugger and the cookie is only null on these 3 users. But the cookie looks the same as the cookie for the many users that login successfully.