select insert('a',2,1,'b') doesn't return expected 'a'

mysql> select insert('a',2,1,'b');

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

| insert('a',2,1,'b') |

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

| ab                  |

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

1 row in set (0.00 sec)

mysql> select version();

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

| version()       |

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

| 10.1.10-MariaDB |

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

1 row in set (0.00 sec)

select insert('a',2,1,'b');