ObservableCollection doesn't update a DataGrid inside a ContentControl

Question

I have an application with a MainWindow that contains a ContentControl, inside of which I put a number of views. One of them contains a DataGrid that is supposed to show some data, which is bound to an ObservableCollection in the ViewModel. I am not able to make the binding work; if I initialize the data in the ViewModel constructor the data get displayed correctly, but if I load them dinamically after application start ("load data" button) the grid just doesn't get updated. Please help I've been busting my head on this for two days now. I already tried the fix suggested here: C#/WPF - View does not update, no known fix works but it doesn' work. On the contrary, it prevents data being loaded also from the ViewModel constructor.

Following is a simplified example just to show my issue.

This is my MainWindow:

<Window x:Class="TestINotifyProperty.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:TestINotifyProperty"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="50"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        
        <ContentControl Grid.Row="1"
                        Grid.Column="1"
                        Margin="10"
                        Content="{Binding CurrentView}"/>
      
        <Button Grid.Column="1" Grid.Row="0" VerticalAlignment="Center" HorizontalAlignment="Center" Content="Load Data" Command="{Binding LoadData}"/>
        <Button Grid.Column="1" Grid.Row="0" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="2,2,100,0" Content="Change View" Command="{Binding ChangeView}"/>
    </Grid>
</Window>

The View with the DataGrid:

<UserControl x:Class="TestINotifyProperty.DemoView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:TestINotifyProperty" 
             d:DataContext="{d:DesignInstance Type=local:ViewModel}"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <UserControl.DataContext>
        <local:ViewModel/>
    </UserControl.DataContext>
    <Grid>
        <StackPanel>
            <TextBlock HorizontalAlignment="Center" VerticalAlignment="Top" Text="Demo View" FontSize="100" FontFamily="courier" Foreground="Blue"/>
            <DataGrid ItemsSource="{Binding Persons}"/>
        </StackPanel>
    </Grid>
</UserControl>

My Model Class:

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace TestINotifyProperty
{
    internal class Model : ObservableObject
    {
        public class Person : INotifyPropertyChanged
        {
            private string surname;

            public string Surname
            {
                get { return surname; }
                set
                {
                    surname = value;
                    OnPropertyChanged();
                }
            }

            private string name;

            public string Name
            {
                get { return name; }
                set { name = value;
                    OnPropertyChanged(); 
                }
            }


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

My ViewModel:

using System.Collections.ObjectModel;

namespace TestINotifyProperty
{
    internal class ViewModel : ObservableObject
     
    {
        public ObservableCollection<Model.Person> persons { get; set; }
        public ObservableCollection<Model.Person> Persons { get { return persons; } }
        public RelayCommand LoadData { get; set; }
        public RelayCommand ChangeView { get; set; }

        public  HomeViewModel HomeVM {get; set;}    
        internal DemoViewModel DemoVM { get; set; }

        private object _currentView;
        public object CurrentView
        {
            get { return _currentView; }
            set
            {
                _currentView = value;
                OnPropertyChanged();
            }
        }

        public ViewModel()
        {
            LoadData = new RelayCommand(o => { Load(); });
            ChangeView = new RelayCommand(o => { changeView();});
            persons = new ObservableCollection<Model.Person>();
            HomeVM = new HomeViewModel();
            DemoVM = new DemoViewModel();
            CurrentView = HomeVM;
            Persons.Add(new Model.Person { Surname= "Doe", Name = "John" });
            Persons.Add(new Model.Person { Surname = "Doe", Name = "Jane" });
            Persons.Add(new Model.Person { Surname = "Doe", Name = "Baby" });
        }

        private void Load()
        {
            CurrentView = DemoVM;
            Persons.Add(new Model.Person { Surname = "Roe", Name = "Richard" });
            Persons.Add(new Model.Person { Surname = "Roe", Name = "Janie" });
        }
       private void changeView()
        {
            if (CurrentView == HomeVM)
                CurrentView = DemoVM;
            else CurrentView = HomeVM;
        }
    }
}

ObservableObject Class (just used to derive classes to implement INotifyPropertyChanged):

using System.ComponentModel;
using System.Runtime.CompilerServices;


namespace TestINotifyProperty
{
    public class ObservableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged([CallerMemberName] string name = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }
    }
}

DemoViewModel class:

namespace TestINotifyProperty
{
    public class DemoViewModel : ObservableObject
    {
    }
}

App.xaml file:

<Application x:Class="TestINotifyProperty.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:TestINotifyProperty"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <DataTemplate DataType="{x:Type local:HomeViewModel}">
            <local:HomeView/>
        </DataTemplate>
        <DataTemplate DataType="{x:Type local:DemoViewModel}">
            <local:DemoView/>
        </DataTemplate>
    </Application.Resources>
</Application>