JSON_OBJECT() doesn't honour datatype in function

DELIMITER //

CREATE OR REPLACE FUNCTION json_function_test ( a INT )

    RETURNS JSON

BEGIN

    DECLARE `_id` INT UNSIGNED;

    DECLARE `_id2` INT UNSIGNED;

    DECLARE `_id3` INT UNSIGNED DEFAULT 9;

    SET @_temp = UNIX_TIMESTAMP();

    SET _id    = UNIX_TIMESTAMP();

    SET _id2   = 444;

    return JSON_OBJECT("argument_int", a, "_id",_id, "_id2",_id2, "_id3", _id3, "temp_var", @_temp) ;

END;//

DELIMITER ;

select json_function_test(123);

{"argument_int": "123", "_id": "1692396866", "_id2": "444", "_id3": "9", "temp_var": "1692396866"}

select JSON_OBJECT("temp_var",@_temp, "a",UNIX_TIMESTAMP());

| {"temp_var": "1692396982", "a": 1692397120}          |

select JSON_OBJECT("temp_var",@_temp, "a",UNIX_TIMESTAMP());

| {"temp_var": 1692397171, "a": 1692397176}            |