Clicking on the 'Synchronize' button on a user object shows the following message:
Client '0000': LDAP variable 'UC_LDAP_DOMAIN' does not contain a 'SYNC_LOGIN' entry.
Release : 21.x
Component : AUTOMATION ENGINE
Configuration. Change in behavior in version 21.0.
Link to the documentation
Additionally, make sure that the variable DOMAIN_ALIAS is also set in the UC_LDAP_* file in case the Domain is not the same as the Department.
This is also needed on v21.
In v21 the LDAP settings once correctly specified , should automatically synchronize without the need to run the LDAP Synchronize button.