The customer is moving contacts from one Active Directory to another in phases and noticed contacts being deactivated by pdm_ldap_import.

If contacts are being moved from one Active Directory to another, it is important to keep the LDAP configuration in the CA Service Desk side updated with the new Active Directory domain information. If the older Active Directory is still configured in the CA Service Desk side, pdm_ldap_import will deactivate contacts because they no longer exist in the older Active Directory.

CA Service Desk 17.1
CA Service Desk 17.0
CA Service Desk 14.1

When  moving contacts from one Active Directory to another, make sure the LDAP configuration is updated with information from the new Active Directory. It will avoid conflicts when running pdm_ldap_import. It creates and updates contacts based on the LDAP configuration. 

See Administration -> Options Manager -> LDAP below:

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Create Contacts in Batch Mode Using LDAP Data