Immersive mode with popup menu Android API's bug?

Immersive(-sticky) mode cannot hide navigation bar completely. When I tap and show popup menu, the navigation bar (with transparent background) is raised like a zombie. This phenomena is same both on API-29 or earlier and on API-30.

Is this API's bug or my code's failure?

Here is a sample code:

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        Toolbar toolbar = findViewById(R.id.toolbar);
        toolbar.inflateMenu(R.menu.main);
    }

    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);

        if (hasFocus) {
            hideSystemUI();
        }
    }

    private void hideSystemUI() {
        Window window = getWindow();
        View decorView = window.getDecorView();

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
            WindowInsetsController windowInsetsController = decorView.getWindowInsetsController();
            windowInsetsController.setSystemBarsBehavior(
                    WindowInsetsController.BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE
            );
            windowInsetsController.hide(
                    WindowInsets.Type.statusBars()
                  | WindowInsets.Type.navigationBars()
            );

            window.setDecorFitsSystemWindows(false);
        } else {
            window.addFlags(
                    WindowManager.LayoutParams.FLAG_FULLSCREEN
                  | WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS
            );
            decorView.setSystemUiVisibility(
                    View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
                  | View.SYSTEM_UI_FLAG_FULLSCREEN
                  | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                  | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                  | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
            );
        }
    }
}

layout:

<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/Theme.AppCompat.Light"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

I've already read an old similar question but it has no answer until now.

I concluded this behavior is intended.

Hiding the navigation bar is limited when the main window is focused. When the popup is activated, like dialogues, it has focus while the main window loses. So the navigation bar reappears.