Dynamic value change and showed in UI, but can't use the property in formatted string format.

I have created a page where I am showing the api fetched data without refreshing the page by implementing the inotifypropertychanged.
When I am using property _bookQuantity its working fine.
But when I am binding the value of property with formatted string, it's not working, means the data is not changing dynamically.
I want to use formatted string for proper format of data showing in the UI.
Any suggestion?

I am attaching the code here:

    public double BookQuantity { get; set; }
    public double _bookQuantity  //This is showing data and changing dynamically. Working fine.
    {
        get{return BookQuantity; }
        set
        {
            BookQuantity = value;
            OnPropertyChanged();}
        }
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public FormattedString FormattedBookQuantity   // This is showing data at initial time, but not changing dynamically.
    {
        get
        {
            FormattedString fs = new FormattedString();
            fs.Spans.Add(StyleClass.GetListLabelNoBold("Book Quantity "));
            fs.Spans.Add(StyleClass.GetListLabelNoBold(_bookQuantity.ToString()));
            return fs;
        }
    }