Nested GROUP_CONCAT in stored function gives "ERROR 1111 (HY000) : Invalid use of group function"

/*

In MariaDB version 10.3.27 and possibly later versions, the below function worked.

But in version 10.5.8, calling the function gives error: "ERROR 1111 (HY000) at line 65: Invalid use of group function".

*/ 

DELIMITER $$

DROP TABLE IF EXISTS t1;

DROP TABLE IF EXISTS t2;

CREATE TABLE t1 (t1_id int primary key, t1_text varchar(100));

CREATE TABLE t2 (t2_id1 int, t2_id2 int, t2_text varchar(100), primary key(t2_id1, t2_id2) );

INSERT t1 VALUES (1, 'Text 1'), (2, 'Text 2');

INSERT t2 VALUES (1, 1, 'Text 11'), (1, 2, 'Text 12'), (2, 1, 'Text 21'), (2, 2, 'Text 22');

-- This query works:

SELECT GROUP_CONCAT(

  (

    SELECT GROUP_CONCAT(t2_text)

    FROM t2

    WHERE t2_id1 = t1_id

  )

) this_works

FROM t1;

DROP FUNCTION IF EXISTS fn_test;

$$

CREATE FUNCTION fn_test() RETURNS varchar(500) READS SQL DATA

RETURN (

  SELECT GROUP_CONCAT(

    (

      SELECT GROUP_CONCAT(t2_text)

      FROM t2

      WHERE t2_id1 = t1_id

    )

  ) x

  FROM t1

);

$$

SELECT fn_test();