MVVM update triggers not handled as expected

I have a simple window with two comboboxes, which I want to automatically select a specific item in the view model.

Combobox1: Projects
Combobox2: Versions

When the first project is selected, I try to set the first version of that project.

public List<Project> Projects
{
    get { return projects; }
    set
    {
        Set(() => Projects, ref projects, value);

        var bm1Project = projects.FirstOrDefault(project => project.Name.Equals("BM1"));
        if (bm1Project != null)
        {
            SelectedProject = bm1Project;
        }
    }
}

private Project selectedProject;
public Project SelectedProject
{
    get { return selectedProject; }
    set
    {
        Set(() => SelectedProject, ref selectedProject, value);
        Versions = selectedProject.Versions;
        SelectedVersion = Versions.FirstOrDefault();
    }
}

private Version selectedVersion;
public Version SelectedVersion
{
    get { return selectedVersion; }
    set
    {
        Set(() => SelectedVersion, ref selectedVersion, value);
        SetIssues();
    }
}

Just for reference, the xaml in the window

<TextBlock Text="Project:" HorizontalAlignment="Right" VerticalAlignment="Center" Padding="0,0,10,0"/>
<ComboBox 
    Grid.Row="0" Grid.Column="1" 
    Width="120" Height="30" 
    ItemsSource="{Binding Projects}" SelectedItem="{Binding SelectedProject}" DisplayMemberPath="Name"/>
<TextBlock Grid.Row="0" Grid.Column="2" Text="Sprint:" HorizontalAlignment="Right" VerticalAlignment="Center" Padding="0,0,10,0"/>
<ComboBox 
    Grid.Row="0" Grid.Column="3" 
    Width="120" Height="30" 
    ItemsSource="{Binding Versions}" SelectedItem="{Binding SelectedVersion}" DisplayMemberPath="Id"/>

While debugging I can see that the code is executed step by step. Still the Version ComboBox is not populated.

When I manually select the project, it works like a charm.

What am I doing wrong?