Why after importing contact records were Addressee field values set to "Customized" for records where Custom Addressee had a null value?
I came across the same issue after upgrading a very old civicrm DB to latest (4.7.27)- i ran the following simple queries to reset to my default IDs (listed at civicrm/admin/options/addressee?reset=1 )

UPDATE civicrm_contact
SET addressee_id = 1
WHERE contact_type = 'Individual';

UPDATE civicrm_contact
SET addressee_id = 2
WHERE contact_type = 'Household';

UPDATE civicrm_contact
SET addressee_id = 3
WHERE contact_type = 'Organization';


I didn't need to retain any existing or "customized" but if one did, adding AND isnull(addressee_id) should work