Assignment to composite type variable fails inside function but running query separately yields correct type & value ?

Hi,

I'm probably just missing some nitty-gritty detail here but I've bashed
by head against this one for ages and still couldn't make sense of it.
This is on PostgreSQL 12.2 (on CentOS 7 but PostgreSQL compiled from
sources):

voip=# CREATE TYPE version_num AS (major bigint, minor bigint);
CREATE TYPE
voip=# CREATE TYPE version_num_text AS (major text, minor text);
CREATE TYPE
voip=#
voip=# CREATE TEMPORARY TABLE test_table ( a_major bigint, a_minor bigint );
CREATE TABLE
voip=# INSERT INTO test_table VALUES (1,1);
INSERT 0 1
voip=#
voip=# CREATE OR REPLACE FUNCTION test_func( _id version_num_text )
voip-# RETURNS void AS
voip-# $BODY$
voip$# DECLARE
voip$#    _myid version_num;
voip$# BEGIN
voip$#         SELECT (t1.a_major, t1.a_minor )::version_num INTO _myid
FROM test_table t1 WHERE (t1.a_major,t1.a_minor)::version_num_text = _id;
voip$# END;
voip$# $BODY$ LANGUAGE plpgsql;
CREATE FUNCTION
voip=#
voip=# SELECT test_func( ('1','1')::version_num_text );
ERROR:  invalid input syntax for type bigint: "(1,1)"
CONTEXT:  PL/pgSQL function test_func(version_num_text) line 5 at SQL
statement
voip=# CREATE TEMPORARY TABLE dbg_table AS SELECT (t1.a_major,
t1.a_minor )::version_num FROM test_table t1 WHERE
(t1.a_major,t1.a_minor)::version_num_text = ('1','1')::version_num_text;
SELECT 1
voip=# \d dbg_table
             Table "pg_temp_189.dbg_table"
 Column |    Type     | Collation | Nullable | Default
--------+-------------+-----------+----------+---------
 row    | version_num |           |          |

Bug or feature ?

Cheers,
Tobias