Cannot use "group by" with "left join"

MariaDB [workspace]> DROP TABLE IF EXISTS t1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [workspace]> DROP TABLE IF EXISTS t2;

Query OK, 0 rows affected (0.00 sec)

MariaDB [workspace]> CREATE TABLE t1(id CHAR(16), PRIMARY KEY(id));

Query OK, 0 rows affected (0.02 sec)

MariaDB [workspace]> INSERT INTO t1 VALUES ('78622');

Query OK, 1 row affected (0.00 sec)

MariaDB [workspace]> INSERT INTO t1 VALUES ('786220');

Query OK, 1 row affected (0.00 sec)

MariaDB [workspace]> CREATE TABLE t2(id CHAR(16), VAR INT DEFAULT NULL, PRIMARY KEY(id));

Query OK, 0 rows affected (0.04 sec)

MariaDB [workspace]> INSERT INTO t2 VALUES ('78622',1);

Query OK, 1 row affected (0.00 sec)

MariaDB [workspace]> SELECT id, VAR FROM t1 LEFT JOIN t2 USING(id) GROUP BY id;

+--------+------+

| id     | VAR  |

+--------+------+

| 78622  | NULL |

| 786220 | NULL |

+--------+------+

2 rows in set (0.00 sec)

MariaDB [workspace]> SELECT id, VAR FROM t1 LEFT JOIN t2 USING(id);

+--------+------+

| id     | VAR  |

+--------+------+

| 78622  |    1 |

| 786220 | NULL |

+--------+------+

2 rows in set (0.00 sec)