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(dot)shinderuk(at)postgrespro(dot)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.