ColdFusion 11 Throws Error Session Variable Undefined, When in fact it is Defined as shown by CFDump.

I am doing isDefined checks for relevant variables when neccessary. The problem is not that the code errors out undefined ... it's that the variable is actually defined and errors out undefined.

As a simplified example;

Login.cfm Form supplies form.username and form.password to login_action.cfm then redirects to index.cfm after succesful LDAP authentication. I will not include all lines as it would be too long.

Login_action.cfm

<cfinclude template="ldap.cfm">
<!--- LDAP login code goes here; checks if credential's supplied are valid and sets variable #logged_in# ... --->

<cftry>
    <cfif isdefined("logged_in") and logged_in>

        <!--- Create Session User Structure and Default Profile --->
        <cfset session.user=StructNew()>
        <cfset session.user.username=form.username>
        <cfset session.user.firstname="">
        <cfset session.user.surname="">
        <cfset session.user.email="">

        <!--- Pull in additional Details --->
        <cfquery name="qDetails" datasource="userdetails">
        SELECT firstname,surname,email FROM Profiles
        WHERE username = '#session.user.username#'
        </cfquery>
        <cfif isdefined("qDetails") and qDetails.recordcount>
            <cfoutput query="qDetails">
            <cfset session.user.firstname ="#qDetails.firstname#">
            <cfset session.user.surname="#qDetails.surname#">
            <cfset session.user.email="#qDetails.email#">
            </cfouput>
        </cfif>
    <cfelse>
        <!--- Throw Error back to login page --->
        <cfthrow message="1" type="Custom_Security" ErrorCode="Fail">
    </cfif>
    <cfcatch type="any">
        <!--- Throw Error back to login page --->
        <cfthrow message="2" type="Custom_Security" ErrorCode="Fail">
    </cfcatch>
</cftry>
<!--- Redirect User to login page after Successful Login --->
<cflocation url="index.cfm">

Index.cfm

<!--- At Top of Index session.user.email always exists, in the event it is undefined it will be set to "" --->
<cfparam name="session.user.email" default="">

<!--- Lots of lines of code above here build HTML page etc <cfoutput> references to #session.user.email# and other session vars --->

<!--- Example pull some more information this line may or may not error... --->
<cfquery name="qUserInformation" datasource="userdetails">
SELECT * FROM Information
WHERE user_email = '#session.user.email#'
ORDER BY SomeOtherColumn
</CFQUERY>
<!--- Example point where a session error can be generated less than 2 lines above it has just finished getting information using session.user.email --->
<cfquery name="qUserGroups" datasource="userdetails">
SELECT * FROM Groups
WHERE user_email = '#session.user.email#'
ORDER BY SomeColumn
</CFQUERY>

<!--- Some other common examples require cfscript - and throw errors; above lines may not ... --->
<cfscript>
oHomePage=CreateObject("component","site_components.homepage");
qNews=oHomePage.getNews("#session.user.category#");
</cfscript>


<!--- More lines to do with HTML Footer etc <cfoutput> more stuff --->

in the wwwroot the Application.cfm handles all errors generated during rendering of the page, which includes generating a cfdump of the session scope.

When I get the Error Log I can see in the cfcatch cfdump the error... Element USER.EMAIL UnDefined in SESSION.

struct

However when I check the SESSION scope which is also dumped ... It is defined!

What is causing this strange and irratic behaviour? The problem is so far I cannot replicate the issue. It is intermittant. The issue only started after upgrading to ColdFusion 11. This code has worked flawlessly for years on CF8.