Stored aggregate function wrong result in correlated subquery

/*

• See https://mariadb.com/kb/en/library/stored-aggregate-functions/
  */

CREATE TABLE marks(stud_id INT, grade_count INT);
INSERT INTO marks VALUES (1,6), (2,4), (3,7), (4,5), (5,8), (5,9);
SELECT * FROM marks;

/* I get:

• 1 6
• 2 4
• 3 7
• 4 5
• 5 8
• 5 9
• Now create the following aggregate function:
  */

DELIMITER //
DROP FUNCTION IF EXISTS aggregate_count //
CREATE AGGREGATE FUNCTION IF NOT EXISTS aggregate_count(x INT) RETURNS INT
BEGIN
DECLARE count_students INT DEFAULT 0;
DECLARE CONTINUE HANDLER FOR NOT FOUND
RETURN count_students;
LOOP
FETCH GROUP NEXT ROW;
IF x THEN
SET count_students = count_students+1;
END IF;
END LOOP;
END //
DELIMITER ;

/* I can use aggregation in my queries as the following: */
select stud_id, aggregate_count(stud_id)
from marks
group by stud_id;
/*

• as expected I get:
  *
• 1 1
• 2 1
• 3 1
• 4 1
• 5 2
  */

/* but when used in a correlated subquery I have wrong result:*/
select t.stud_id, (
select aggregate_count(stud_id)
from marks
where stud_id = t.stud_id
) c
from (
select 1 as stud_id
union select 6
union select 5
) t;

/*

• I get:
• 1 1
• 6 1 <---- wrong
• 5 2
• the correct value should be 0.
• I debug a bit the aggregation function:
  */
  CREATE TABLE debug(msg TEXT);
  DELIMITER //
  DROP FUNCTION IF EXISTS aggregate_count //
  CREATE AGGREGATE FUNCTION IF NOT EXISTS aggregate_count(x INT) RETURNS INT
  BEGIN
  DECLARE count_students INT DEFAULT 0;
  DECLARE CONTINUE HANDLER FOR NOT FOUND
  RETURN count_students;
  LOOP
  FETCH GROUP NEXT ROW;
  IF x THEN
  SET count_students = count_students+1;
  END IF;
  insert into `debug`(msg) VALUES (concat('value: ', ifnull(x, 'null'), " -> count_students: ", count_students));
  END LOOP;
  END //
  DELIMITER ;

select t.stud_id, (
select aggregate_count(stud_id)
from marks
where stud_id = t.stud_id
) c
from (
select 1 as stud_id
union select 6
union select 5
) t;

select * from debug;

/*

• value: 1 -> count_students: 1
• value: null -> count_students: 0
• value: 5 -> count_students: 1
• value: 5 -> count_students: 2
• Function seems to be invoked and return the correct value, but query return a wrong value.
  */