MariaDB [test]> CREATE TABLE t1 (a varchar(5));

Query OK, 0 rows affected (0.035 sec)

MariaDB [test]> INSERT INTO t1 VALUES ('a'),('ä');

Query OK, 2 rows affected (0.014 sec)

MariaDB [test]> select * from t1;

+------+

| a    |

+------+

| a    |

| ä    |

+------+

2 rows in set (0.000 sec)

MariaDB [test]> SELECT JSON_ARRAYAGG(a) FROM t1;

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

| JSON_ARRAYAGG(a) |

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

| ["a","▒"]         |

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

1 row in set (0.001 sec)

MariaDB [test]> SELECT JSON_ARRAY(a) FROM t1;

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

| JSON_ARRAY(a) |

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

| ["a"]         |

| ["ä"]         |

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

2 rows in set (0.000 sec)

MariaDB [test]> select @@version;

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

| @@version                            |

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

| 10.5.8-MariaDB-1:10.5.8+maria~bionic |

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

MariaDB [(none)]> select @@character_set_server,@@character_set_client,@@character_set_results,@@collation_connection;

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

| @@character_set_server | @@character_set_client | @@character_set_results | @@collation_connection |

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

| utf8mb4                | utf8                   | utf8                    | utf8_general_ci        |

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

1 row in set (0.000 sec)