How do CHECK Constraint Function privileges work?

CREATE SCHEMA app_public;

CREATE SCHEMA app_private;

GRANT USAGE ON SCHEMA app_public TO "grant_test_role";

CREATE OR REPLACE FUNCTION app_private.constraint_max_length(name_ TEXT) RETURNS BOOLEAN
  AS $$
BEGIN
  -- do some checks here
  RETURN TRUE;
END;
$$ LANGUAGE plpgsql;

CREATE TABLE app_public.test_tab (
   id INT NOT NULL PRIMARY KEY,
   name TEXT NOT NULL,

   CONSTRAINT name_length_check CHECK (app_private.constraint_max_length(name));
);

BEGIN;
  SET LOCAL ROLE TO "grant_test_role";
  INSERT INTO app_public.test_tab (id, name) VALUES (1, 'Very Long Name');
COMMIT;

My expectation was the INSERT would give me an exception due to "grant_test_role" not having permissions on the "app_private" schema, but it does not. Why does the CHECK constraint function executes fine in this instance?

I feel I'm missing some knowledge on how PostgreSQL internals work when checking privileges for CHECK constraint expressions, and I didn't find anything mentioned about this in documentation.

There are some followup questions regarding the PUBLIC role, which I will reserve until I can get some clarification on the current behavior of the CHECK constraint function's privilege check.

Thanks & Kind Regards,

/Ruwan.