Foundry JSON troubles

FileUtils.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity >=0.8.4;

import { Script } from "forge-std/Script.sol";
import { console2 } from "forge-std/console2.sol";
import { stdJson } from "forge-std/StdJson.sol";

contract FileUtils is Script {
    using stdJson for string;

    function serializeDemo() public {

        string memory json = "INTERNAL_KEY";
        console2.log(json); // output: INTERNAL_KEY
        json.serialize("address1", address(1));
        console2.log(json); // output: INTERNAL_KEY
        json = json.serialize("address2", address(2));
        console2.log(json); // output: {"address2":"0x0000000000000000000000000000000000000002","address1":"0x0000000000000000000000000000000000000001"}
        json = json.serialize("address3", address(3));
        console2.log(json); // output: {"address3":"0x0000000000000000000000000000000000000003"}
        // shouldn't the above output include all three address entries?
    }
}