JSON_MERGE merges valid JSON objects incorrectly

MySQL [test]> SELECT JSON_MERGE('{"a":"b"}','{"c":"d"}');

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

| JSON_MERGE('{"a":"b"}','{"c":"d"}') |

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

| {"a": "b", "c": "d"}                |

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

1 row in set (0.00 sec)

MariaDB [test]> SELECT JSON_MERGE('{"a":"b"}','{"c":"d"}');

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

| JSON_MERGE('{"a":"b"}','{"c":"d"}') |

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

| ["{\"a\":\"b\"}", "{\"c\":\"d\"}"]  |

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

1 row in set (0.00 sec)