Window opacity not being effected in a method?

public partial class overlay : Window, INotifyPropertyChanged
    {
        public static overlay? thisOverlay;
        DoubleAnimation fade = new DoubleAnimation(0, new Duration(TimeSpan.FromSeconds(5)));

        public event PropertyChangedEventHandler? PropertyChanged;

        private double _windowOpacity;
        public double WindowOpacity
        {
            get => _windowOpacity;
            set
            {
                if (_windowOpacity == value)
                    return;

                _windowOpacity = value;
                OnPropertyChanged();
            }
        }

        public overlay()
        {
            InitializeComponent();
            this.Loaded += Overlay_Loaded;
            WindowOpacity = 0.2;
        }

        private void Overlay_Loaded(object sender, RoutedEventArgs e)
        {
            thisOverlay = this;

        }
        protected void OnPropertyChanged([CallerMemberName] string name = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }

        public void setOpacity() // Function is ran outside of this class
        {
            this.WindowOpacity = 0.5; // This doesn't work
            Trace.WriteLine("Works"); // This prints
        }