When running this query:

select json_object('x', json_arrayagg(json_object('a', 1)));

I'm getting this content (with the JSON_ARRAYAGG content being embedded in a string).

{"x": [{"a": 1}]}

I'm expecting this instead:

{"x": "[{\"a\": 1}]"}

I'm getting the expected result in MySQL or in Oracle. In the case of Oracle, use this syntax for comparison:

select json_object('x' value json_arrayagg(json_object('a' value 1))) from dual;