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The following bug has been logged on the website:

Bug reference:      18656
Logged by:          Alexander Alehin
Email address:      panso8@gmail.com
PostgreSQL version: 16.4
Operating system:   Debian 11
Description:        

CREATE TABLE stbl_states(
  id    int primary key,
  state text
);

CREATE TABLE stbl_logs(
  ts timestamp default clock_timestamp(),
  log text
);

CREATE or replace FUNCTION stbl_get_state(p_id int) RETURNS text
  LANGUAGE sql STABLE AS
$$
select coalesce(k.state, 'off') from stbl_states k where k.id = p_id;
$$;

CREATE or replace PROCEDURE stbl_log(p_message text)
  LANGUAGE plpgsql AS
$$
begin
  insert into stbl_logs(log) values (p_message);
end;
$$;

CREATE or replace PROCEDURE stbl_update_state()
  LANGUAGE plpgsql AS
$$
declare
  s text;
begin
  update stbl_states set state='on' where id=1;
  s := stbl_get_state(1);
  call stbl_log(s);
  call stbl_log(stbl_get_state(1));
  insert into stbl_logs(log) values (stbl_get_state(1));
exception when others then
  s := SQLERRM;
  call stbl_log(s);
end;
$$;

insert into stbl_states(id) values(1);
call stbl_update_state();
select log from stbl_logs order by ts;

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

 log
-----
 on
 off
 on
(3 rows)

But expected:
on
on
on

If "STABLE" or "exception ..." is removed, it will work as expected.