Group concat returns incorrect result when using DISTINCT and ORDER BY

create table t1 (a varchar(1000), b varchar(1000), c varchar(1000));

insert t1 select seq,seq, seq from seq_1_to_9;

insert t1 select seq,seq, seq from seq_1_to_9;

set tmp_table_size=1024; // set so that unique tree is dumped to the disk

MariaDB [test]> select group_concat(c order by a,b) from t1;

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

| group_concat(c order by a,b)        |

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

| 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9 |

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

1 row in set (0.005 sec)

MariaDB [test]> select group_concat(distinct c order by a,b) from t1;

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

| group_concat(distinct c order by a,b) |

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

| 1                                     |

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

1 row in set (0.006 sec)