ADD FOREIGN KEY fails, but the records exist

Postgresql 12.5

It's a self-referential FK on a single (but partitioned) table.  The ALTER
TABLE command fails, but I queried it, and the record that it fails on
exists.  I modified the original INITIALLY IMMEDIATE clause to INITIALLY
DEFERRED but that did not help.

What am I doing wrong?

(We're migrating from Oracle to PostgreSQL, and this is working in Oracle.)

sides=> ALTER TABLE employer_response
    ADD CONSTRAINT amended_response_fk FOREIGN KEY (amended_response_id,
part_date)
        REFERENCES employer_response(employer_response_id, part_date)
    ON DELETE NO ACTION DEFERRABLE INITIALLY DEFERRED ;
ERROR:  insert or update on table "employer_response_p2021_01" violates
foreign key constraint "amended_response_fk"
DETAIL:  Key (amended_response_id, part_date)=(103309154, 2021-01-06
00:00:00) is not present in table "employer_response".
sides=>
sides=> select employer_response_id, part_date
sides-> from strans.employer_response
sides-> where amended_response_id = 103309154;
employer_response_id |      part_date
----------------------+---------------------
            103309156 | 2021-01-06 00:00:00
(1 row)

sides=> select employer_response_id, part_date
from strans.employer_response_p2021_01
where amended_response_id = 103309154;
employer_response_id |      part_date
----------------------+---------------------
            103309156 | 2021-01-06 00:00:00
(1 row)

--
Angular momentum makes the world go 'round.