"permission denied to COPY to or from an external program" even with GRANT pg_execute_server_program

Dear Postgreezers,

been banging my head against this one for a couple days. Googling and
StackExchange were just as useful, so you're my last hope. I've been
unable to get a non-admin user to run Copy From Program even after granting
pg_execute_server_program, and everything else I could think of. It always
fails with ERROR: permission denied to COPY to or from an external program.

I'll let the code speak by itself. Here's a minimal example that I've
tried in the last official Docker image:

-- Spin a temporal Pg and connect to psql

--docker run --name pg16 -e POSTGRES_PASSWORD=qwer -d postgres:16

--docker exec -ti pg16 psql -U postgres

CREATE TABLE testtable (

id int NOT NULL GENERATED ALWAYS AS IDENTITY,

name text NOT NULL

);

Create Role justintestin noinherit login password 'qwer';

-- Necessary privileges

GRANT CONNECT ON DATABASE postgres TO justintestin;

GRANT USAGE ON SCHEMA public TO justintestin;

GRANT ALL ON ALL TABLES IN SCHEMA public TO justintestin;

-- Apply them to new tables/views created by admin account

ALTER DEFAULT IN SCHEMA public GRANT ALL ON TABLES TO justintestin;

-- Allow Copy From Program... or try to anyway

GRANT pg_execute_server_program TO justintestin;

-- Tests

GRANT ALL ON testtable TO justintestin;

GRANT ALL ON SCHEMA public TO justintestin;

GRANT ALL ON DATABASE postgres to justintestin;

GRANT pg_read_all_data TO justintestin;

GRANT pg_write_all_data TO justintestin;

--Copy works with admin account

Copy testtable(name) From Program 'echo "Buffa Testata"' CSV;

-- COPY 1

--But fails with justintestin

SET role justintestin;

Copy testtable(name) From Program 'echo "Errato Denegato"' CSV;

--SQL Error [42501]: ERROR: permission denied to COPY to or from an
external program

-- Detail: Only roles with privileges of the "pg_execute_server_program"
role may COPY to or from an external program.

--Even tho he's privileged

SELECT rolname FROM pg_roles WHERE

pg_has_role(current_user, oid, 'member');

-- rolname

---------------------------

-- pg_read_all_data

-- pg_write_all_data

-- pg_execute_server_program

-- justintestin

--Insert works

Insert Into testtable ("name") VALUES('Pazzo Intestinato');

--INSERT 0 1

Select * From testtable;

SELECT current_user, session_user;

-- Clean up for new test

SET role postgres;

Drop Table testtable;

Drop Owned By justintestin;

Drop Role justintestin;

What am I missing? (besides a few chunks of hair)