rowtype cache on trigger

gistfile1.sql

CREATE TABLE foo(
  id integer
);

CREATE FUNCTION foo_trigger() RETURNS trigger LANGUAGE plpgsql AS $_$
DECLARE
  r foo%rowtype;
BEGIN
  SELECT NEW.* INTO r;
  RETURN r;
END;
$_$;

CREATE TRIGGER foo AFTER INSERT ON foo FOR EACH ROW EXECUTE PROCEDURE foo_trigger();

INSERT INTO foo VALUES(1);

ALTER TABLE foo ADD COLUMN flag boolean;

INSERT INTO foo VALUES(2);

DROP TABLE foo;
DROP FUNCTION foo_trigger();

results in
ERROR: returned row structure does not match the structure of the triggering table
DETAIL: Number of returned columns (1) does not match expected column count (2).
CONTEXT: PL/pgSQL function foo_trigger() during function exit