modifyTimeStamp not modified on replica

Hello

when I modify the mail attribute on the master, the attribute is correctly replicated on the replica, but the modifyTimeStamp isn't !? shouldn't it be modified !?

The master is runing openldap-servers-2.3.43-12.el5_7.10 the replica that failes to replicate modifyTimeStamp is an openldap-servers-2.4.23-20.el6.i686 on a openldap-servers-2.3.43-12.el5_7.10 replica, modifyTimeStamp is correctly updated ! could it be a openldap version incompatibility ? or perhaps an ACL miss-configuration !?

Thanks for your help .