Do manual HAProxy:
It is important to note that as long as HAProxy does not support keep-alive connections, only the first request of a connection will receive the header. For this reason, it is important to ensure that "option httpclose" is set when using this option.
Examples : # Public HTTP address also used by stunnel on the same machine frontend www mode http option forwardfor except 127.0.0.1 # stunnel already adds the header
# Those servers want the IP Address in X-Client backend www mode http option forwardfor header X-Client
See also : "option httpclose"
Então você pode tentar a opção httpclose, mas eu tentaria depois de horas ou em teste no caso de uma penalidade de desempenho?