Using JSON_KEY_VALUE in subquery truncates the result

Hi,

I found that when using the JSON_KEY_VALUE function in a subquery truncates the result, this test case can reproduce this:

SELECT JSON_KEY_VALUE('{"a":1}', '$') AS direct_result; -- [{"key": "a", "value": 1}]

SELECT x, LENGTH(x) AS len FROM (SELECT JSON_KEY_VALUE('{"a":1}', '$') AS x) AS dt; -- [{"key": "a",   14

I can reproduce this with the commit version f6e44c1b7819f9befa3824f07866822288231528 of MariaDB server in github repo.