LDAP searches don't return a cn attribute
If you add a Person document to the Domino Directory without using Notes registration, and you enter a hierarchical name in the FullName (User name) field, the leftmost part of the distinguished name does not automatically become the cn (common name) attribute value. You must add the common name as a 2nd value in the FullName field to define a cn attribute for the entry.

Person documents created through Notes registration automatically have a 2nd value added to the FullName field to define the cn attribute.

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