Does F5 automatically delete cookies in the header?

Right - so it's the responses where you are losing cookies? It's gotta be the iRule doing it and 2 is prime suspect but I'm scratching my head to see exactly what is wrong. Add these extra statements in;-

when HTTP_RESPONSE {
    set ck [HTTP::header values "Set-Cookie"]
    log local0. $ck
    log local0.[HTTP::cookie names]
    HTTP::header remove "Set-Cookie"
    foreach acookie $ck {
        log local0. "Cookie $acookie"
        HTTP::header insert "Set-Cookie" "${acookie}; HttpOnly"
    }
}