Combobox displays ValueMember instead of DisplayMember - winforms

Question

I have a simple class User like below:
public class User
{
public int UserId { get; set; }
public string Name { get; set; }
public string Surname { get; set; }
public string FullName
{
get
{
return Name + " " + Surname;
}
}
}
Then I have form frmProcess, which displays list of Users in combobox, like below. The problem is, that if I set DisplayMember to FullName then what is displayed is UserId column. What's weird is that if I set DisplayMember to Surname, then Surname is displayed. What am I doing wrong?
public partial class frmProcess : Form
{
List<User> Users;
private async void frmProcess_Load(object sender, EventArgs e)
{
Users = new List<User>();
User A = new User { UserId = 1, Name = "Michael", Surname = "Smith" };
User B = new User { UserId = 2, Name = "John", Surname = "Johnson" };
Users.Add(A);
Users.Add(B);
cmbStartedBy.DataSource = Users;
cmbStartedBy.DisplayMember = "FullName";
cmbStartedBy.ValueMember = "UserId";
}
}

Answer 1

I have run your exact code in a winforms application in VS2013 and get this:
EDIT: The only difference is the async on Form load.
Entire code:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace WindowsFormsApplication2
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
List<User> Users;
private void Form1_Load(object sender, EventArgs e)
{
Users = new List<User>();
User A = new User { UserId = 1, Name = "Michael", Surname = "Smith" };
User B = new User { UserId = 2, Name = "John", Surname = "Johnson" };
Users.Add(A);
Users.Add(B);
comboBox1.DataSource = Users;
comboBox1.DisplayMember = "FullName";
comboBox1.ValueMember = "UserId";
}
}
public class User
{
public int UserId { get; set; }
public string Name { get; set; }
public string Surname { get; set; }
public string FullName
{
get
{
return Name + " " + Surname;
}
}
}
}

Answer 2

Ok, found the bug. When I was posting my question I simplified my class and removed, among others, the data annotations. Surprisingly, this made the code work correctly. As it turns out, the problem was caused by FullName property being marked as [Browsable(false)]. I marked it unbrowsable so that this property wasn't displayed in DataGridViews, didn't realize the impact is also on comboboxes.
public class User
{
[DisplayName("ID")]
public int UserID { get; set; }
[DisplayName("Imie")]
[Required(AllowEmptyStrings = false, ErrorMessage = "Pole imie nie może być puste!")]
public string Name { get; set; }
[DisplayName("Nazwisko")]
[Required(AllowEmptyStrings = false, ErrorMessage = "Pole nazwisko nie może być puste!")]
public string Surname { get; set; }
[Browsable(false)]// <-- this was causing the issue
public string FullName
{
get
{
return Name + " " + Surname;
}
}
}