MariaDB [test]> SET @j = '["a", {"b": [1, 2]}, [3, 4]]';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> SELECT JSON_ARRAY_INSERT(@j, '$[1].b[0]', 'x');

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

| JSON_ARRAY_INSERT(@j, '$[1].b[0]', 'x') |

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

| NULL                                    |

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

1 row in set (0.01 sec)

MySQL [test]> SET @j = '["a", {"b": [1, 2]}, [3, 4]]';

Query OK, 0 rows affected (0.00 sec)

MySQL [test]> SELECT JSON_ARRAY_INSERT(@j, '$[1].b[0]', 'x');

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

| JSON_ARRAY_INSERT(@j, '$[1].b[0]', 'x') |

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

| ["a", {"b": ["x", 1, 2]}, [3, 4]]       |

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

1 row in set (0.00 sec)