BUG #17669: Invalid TOAST pointer in PL/pgSQL variable

The following bug has been logged on the website:

Bug reference:      17669
Logged by:          Sergey Shinderuk
Email address:      s.shinderuk@postgrespro.ru
PostgreSQL version: 15.0
Operating system:   Ubuntu 22.04
Description:

Is it a bug or a known limitation?

create table t (a int, b text);
alter table t alter column b set storage external;
insert into t values (1, repeat('a',3000));

create function f() returns text as $$
declare
  s text;
begin
  select b into s from t where a = 1;
  truncate t;
  return s;
end;
$$ language plpgsql;

postgres=# select f();
ERROR:  missing chunk number 0 for toast value 24727 in pg_toast_24722

If I replace "truncate" with "drop table", then I get:
ERROR:  could not open relation with OID 24725

With "delete from t" it just works.