TestPage.cs

using System;
using Xamarin.Forms;
using System.Collections.ObjectModel;
using System.Threading.Tasks;

namespace ListViewLayoutBug
{
    public class TestPage : ContentPage
    {
        public TestPage()
        {
            var source = new ObservableCollection<string>();

            var list = new ListView
            {
                // Comment this line out to observe the bug. It's just here to show that the asynchronous
                // loading itself works.
                HeightRequest = 80,
                ItemsSource = source
            };

            LoadItems(source);

            var layout = new AbsoluteLayout();
            layout.Children.Add(list);

            Content = layout;
        }

        async void LoadItems(ObservableCollection<string> items)
        {
            // If you comment the delay out, the function will run synchronously and the content will appear.
            await Task.Delay(2000);

            items.Add("hello");
        }
    }

}