ObservableCollection in ListView - is this a BUG, or how do I bind to the KEY?

I've got the following listview.

        var headerTemplate = new DataTemplate(typeof (ModuleMediaListHeaderTemplate));
        headerTemplate.CreateContent();

        var itemTemplate = new DataTemplate(typeof (ModuleMediaListItemTemplate));
        itemTemplate.CreateContent();

        _listView = new ListView
        {
            IsGroupingEnabled = true,
            HasUnevenRows = false,
            GroupHeaderTemplate = headerTemplate,
            ItemTemplate = itemTemplate
        };
        _listView.SetBinding(ListView.ItemsSourceProperty, "MediaSections");
        _listView.ItemSelected += async (sender, args) => await TriggerMedia((MediaViewModel) args.SelectedItem);

my ModuleMediaListHeaderTemplate is super simple

public class ModuleMediaListHeaderTemplate : ViewCell
{
    public ModuleMediaListHeaderTemplate()
    {
        var title = new Label { TextColor = Color.White, YAlign = TextAlignment.Center };
        title.SetBinding(Label.TextProperty, new Binding("Key", BindingMode.Default, new UppercaseValueConverter()));

        View = new StackLayout
        {
            Padding = new Thickness(8, 0),
            BackgroundColor = Color.FromHex(Colors.br_dark_royal_blue),
            Orientation = StackOrientation.Horizontal,
            Children = { title },
        };
    }
}

And my ViewModel is responsible for taking an IGrouping list and converting it to an ObservableCollection.

    public ObservableCollection<IGrouping<string, MediaViewModel>> MediaSections { get; set; }

    public IList<MediaViewModel> Media
    {
        set
        {
            MediaSections = value.OrderBy(o => o.Number)
                .GroupBy(x => x.ModuleSection.Name)
                .ToObservableCollection();

            OnPropertyChanged();
            OnPropertyChanged(() => MediaSections);
        }
    }

ToObservableCollection is a simple extension method

    public static ObservableCollection<T> ToObservableCollection<T>(this IEnumerable<T> collection)
    {
        return collection != null ? new ObservableCollection<T>(collection) : null;
    }

My problem is that when I run in DEBUG mode, the Header Binding works properly, HOWEVER, when in Release mode, it cannot find the property "Key" for this binding

        title.SetBinding(Label.TextProperty, new Binding("Key", BindingMode.Default, new UppercaseValueConverter()));

is this the nature of converting an IGrouping to an ObservableCollection, or is this a bug?