.NET 4.5 をターゲットにした時のみ、private setter プロパティに TwoWay Binding できる問題

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <TextBox Text="{Binding Hoge, Mode=TwoWay}"/>
        <TextBox Text="{Binding Hoge, Mode=TwoWay}"/>
    </StackPanel>
</Window>

MainWindow.xaml.cs

using System.ComponentModel;
using System.Diagnostics;
using System.Windows;

namespace WpfApplication1
{
    /// <summary>
    /// MainWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private string _Hoge;

        public string Hoge
        {
            get { return _Hoge; }
            private set
            {
                _Hoge = value;
                Debug.WriteLine("Hoge Changed");
                this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(this.Hoge)));
            }
        }

        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
            this.Hoge = "Hoge!";
        }

        public event PropertyChangedEventHandler PropertyChanged;
     }
}