iRule not working with 11.4

Move you HTTP::release command to the CLIENTSSL_CLIENTCERT event.

when CLIENTSSL_CLIENTCERT { 
    if { [SSL::cert count] > 0 } {
        set ClientCertificatePresent 1 
        set IVUSER [substr [X509::subject [SSL::cert 0]] 3 ","]
        HTTP::release
    } 
}
when HTTP_REQUEST { 
    set CookiePresent 0 
    set ClientCertificatePresent 0

    if { [HTTP::cookie exists "IV-USER"] } { 
        set CookiePresent 1 
    }

    if { not [matchclass [IP::remote_addr] equals CompanyX_ACME_NonClientCertificate ] } {
        if { [matchclass [HTTP::uri] starts_with CompanyX.ACME.SSO] } {
            HTTP::collect
            SSL::authenticate once
            SSL::authenticate depth 9
            SSL::cert mode request
            SSL::renegotiate
        }
    }
}
when HTTP_REQUEST_SEND {
    clientside { 
        if { $CookiePresent eq 0 and $ClientCertificatePresent eq 1 } {
            HTTP::cookie insert name "IV-USER" value $IVUSER 
        } 
    } 
}
when HTTP_RESPONSE {
    if { $CookiePresent eq 0 and $ClientCertificatePresent eq 1 } { 
        HTTP::header insert Set-Cookie "IV-USER=$IVUSER; Path=/" 
    } 
}