putStringSet() Issue in Shared Preferences SOLVED

MainActivity.java

//What the problem? 
//If you will put the String Set to Shared Preferences with a putStringSet() method it will be put,
//but for second iteration and more it is work uncorrectly. Let's see
public class MainActivity extends AppCompatActivity {

    private Button mFirstButton, mSecondButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initViews();

        mFirstButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                saveFirst(this);
            }
        });

        mSecondButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                saveSecond(this);
            }
        });

    }

    private void initViews() {
        mFirstButton = findViewById(R.id.first_btn);
        mSecondButton = findViewById(R.id.second_btn);
    }

    private void saveFirst(Context context) {

        Set<String> stringSet = new LinkedHashSet<>();
        stringSet.add("hello1");
        stringSet.add("hello2");

        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(context);
        SharedPreferences.Editor editor = sharedPreferences.edit();
        editor.putStringSet("set1", stringSet);
        editor.apply();

    }

    private void saveSecond(Context context) {

        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(context);
        Set<String> stringSet = sharedPreferences.getStringSet("set1", new LinkedHashSet<>());
        stringSet.add("hello3");

        SharedPreferences.Editor editor = sharedPreferences.edit();
        editor.putStringSet("set1", stringSet);
        editor.apply();
        
    }

}

preferences.xml

<!--Shared Prefences after clicking to First then Second button-->
<?xml version='1.0' encoding='utf-8' standalone='yes' ?>
<map>
    <set name="set1">
        <string>hello1</string>
        <string>hello2</string>
    </set>
</map>
//As you can see, <string>hello3</string> record is absent. So, what to do?

//SOLUTION:
//replace saveSecond to 
 private void saveSecond(Context context) {

        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(context);
        Set<String> stringSet = sharedPreferences.getStringSet("set1", new LinkedHashSet<>());
        Set<String> newStringSet = new LinkedHashSet<>(stringSet); //crutch. if you don't create a new instance of set it will not work :(
        newStringSet.add("hello3");

        SharedPreferences.Editor editor = sharedPreferences.edit();
        editor.putStringSet("set1", newStringSet);
        editor.apply();

    }