Unity JsonUtility Serializer bug with Quaternions (formatting?)

DummyData..cs

using System;
using UnityEngine;
using System.Collections;
using System.IO;

[Serializable]
public class DummyData : ISaveData
{

    [SerializeField] private Vector3 _position = new Vector3();
    [SerializeField] private Quaternion _rotation = new Quaternion(); // <----------- this sometimes doesn't get serialized properly!

    private static FileStream _fs;
    private static StreamWriter _writer;

    public bool SavePosition(DummyDataHolder holder)
    {
        Transform t = holder.transform;

        _position = t.position;
        _rotation = t.rotation;

        return true;
    }

    public string OutputJsonData()
    {
        return JsonUtility.ToJson(this);
    }

    public static bool WriteJsonFile(string data, string path)
    {
        // nothing to write
        if (data == "" || path == "")
            return false;

        using (_fs = new FileStream(path, FileMode.OpenOrCreate))
        {
            using (_writer = new StreamWriter(_fs))
            {
                _writer.Write(data);
            }
        }

        // successfully wrote the file
        return true;
    }
}

DummyData.js

{"_position":{"x":41.80579376220703,"y":43.82134246826172,"z":51.85480499267578},"_rotation":{"x":0.03028147667646408,"y":0.030189329758286477,"z":0.030189301818609239,"w":0.9986291527748108}}}

DummyDataHolder.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class DummyDataHolder : MonoBehaviour
{
    public bool ShouldRotate = true;

    private DummyData _data;
    void Start()
    {
        _data = LoadPreviousData();

        if (_data== null)
            _data = new DummyData();

        StartCoroutine(AutoSave(_data));
    }


    IEnumerator AutoSave(DummyData data)
    {
        WriteDataToJson(data);
        _data = LoadPreviousData();

        yield return new WaitForSeconds(0.2f);
        StartCoroutine(AutoSave(data));
    }

    void WriteDataToJson(DummyData data)
    {
        data.SavePosition(this);
        string json = data.OutputJsonData();

        // nothing to save
        if (json == "")
            return;

        if (DummyData.WriteJsonFile(json, GetJsonPath()))
        {
            string saveMessage = string.Format("Saved dummy data: {0}", json);
            Debug.Log(saveMessage);
        }
    }

    string GetJsonPath()
    {
        return string.Format("{0}{1}",
            "Assets/Resources/DummyData",
            ".json");
    }

    private void Update()
    {
        // the serialization bug happens when the transform's rotation starts changing!
        if (ShouldRotate)
            transform.Rotate(new Vector3(10,10,10) * Time.deltaTime, Space.Self);
    }

    DummyData LoadPreviousData()
    {
        DummyData data = null;
        string json;

        json = DataSaver.RetriveText(GetJsonPath());
        if (json != "")
        {
            Debug.Log("Loaded");
            data = JsonUtility.FromJson<DummyData>(json);
        }

        return data;
    }
}