After migration, the LDAP calendar search is enabled, but the calendar search dialog does not return any results or returns only partial results.
Enabling the LDAP calendar search allows Calendar Server to search (&(objectclass=icscalendaruser)(icscalendarowned=*substr*)).
Manually run two different searches on the LDAP data with the following filters and compare the output:
LDAP search with filter (&(objectclass=icscalendaruser)(icscalendarowned=*substr*))
LDAP search with filter (icscalendarowned=*substr*)
Since the server uses the filter that includes icsCalendarUser object class, the LDAP server might have been deployed with the schema check disabled, and some calendar entries may have been provisioned without the icsCalendarUser object class.