javascript changes wrong object key

Question

below is my code i am defining an object, and adding second and third keys to it. And at the end when i change the third key, it also changes the second key somehow, you can look results from browsers localstorage tab

window.onload = function() {
    localStorage.clear();
    const myObject = {
        "1": {
            keyframes: [
                {"posX": "0", "posY": "0"},
                {"posX": "150", "posY": "0"},
                {"posX": "300", "posY": "0"}
            ]
        }
    }
    // example keyframes for new object key
    let example_keyframes = {
        keyframes: [
            {"posX": "0", "posY": "0"},
            {"posX": "400", "posY": "0"},
            {"posX": "100", "posY": "0"}
        ]
    };
    // adding new key(second) to my object
    let lastkey = Object.keys(myObject)[Object.keys(myObject).length - 1] ;
    myObject[parseInt(lastkey)+1] = example_keyframes;
    window.localStorage.setItem("myObject", JSON.stringify(myObject));

    // adding new key(third) to my object
    lastkey = Object.keys(myObject)[Object.keys(myObject).length - 1] ;
    myObject[parseInt(lastkey)+1] = example_keyframes;
    window.localStorage.setItem("myObject", JSON.stringify(myObject));

    // change third keys, keyframe value
    myObject[3]['keyframes'][2]['posY'] = 12;
    myObject[3]['keyframes'][2]['posX'] = 49;
    window.localStorage.setItem("myObject", JSON.stringify(myObject));
}

i tried to shorten the code and reproduce the error and it's still valid.