Index created in BEFORE trigger not updated during INSERT

Not that it is a useful use case, but I believe that this is
a bug that causes index corruption:

CREATE TABLE mytable(
id integer PRIMARY KEY,
id2 integer NOT NULL
);

CREATE FUNCTION makeindex() RETURNS trigger
LANGUAGE plpgsql AS
$$BEGIN
CREATE INDEX ON mytable(id2);
RETURN NEW;
END;$$;

CREATE TRIGGER makeindex BEFORE INSERT ON mytable FOR EACH ROW
EXECUTE PROCEDURE makeindex();

INSERT INTO mytable VALUES (1, 42);

SELECT * FROM mytable;
┌────┬─────┐
│ id │ id2 │
├────┼─────┤
│ 1 │ 42 │
└────┴─────┘
(1 row)

But 42 is not present in the index:

SET enable_seqscan = off;

SELECT * FROM mytable WHERE id2 = 42;
┌────┬─────┐
│ id │ id2 │
├────┼─────┤
└────┴─────┘
(0 rows)