Knew I was forgetting something :) Here's the overlay info from the slave:
overlay chain chain-uri "ldap://172.25.1.2 chain-idassert-bind bindmethod="simple"
binddn="cn=ldapChain,o=partner_x,dc=example,dc=net" credentials="secret" mode="none" # mode="self"
The documentation I pointed you to clearly shows that you need to use mode="self". Please see slapd-ldap(5) for details on the meaning of those parameters. Enabling mode="self" requires the provider to be able to deal with the proxied authorization control (RFC 4370) in requests.
p.