Why is SingleOrDefault not returning the whole entity graph

I am trying to get the whole graph of an entity User. The User class is:

Updated

public class User
{
    [Key]
    public int Id { get; set; }

    public string email_address { get; set; }

    [Column("Password Hash")]
    public string PasswordHash { get; set; }

    public string custnmbr { get; set; }        

    public Customer Customer { get; set; }
}

My Customer Class:

public class Customer
{
    [Key]
    public string Custnmbr { get; set; }

    public string Custname { get; set; }        

    public ICollection<User> Users { get; set; }
}

I have this in my DbContext:

public class EntityContext : DbContext
{
    public EntityContext(DbContextOptions<EntityContext> options)
        : base(options)
    {
    }

    public DbSet<User> Users { get; set; }
    public DbSet<Customer> Customers { get; set; }
    
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
            .HasOne<Customer>(u => u.Customer)
            .WithMany(c => c.Users)
            .HasForeignKey(u => u.custnmbr);
    }
}

My UserService:

public class UserService : IUserService
{
    private readonly EntityContext _context;
            
    public UserService(EntityContext context)
    {
        _context = context;
        _context.Users.Include("Customer").Load();
    }

    public IEnumerable<User> GetAll()
    {
        var users = _context.Users.Local.ToList();
        return users;
    }

    public User GetByUsername(string emailAddress)
    {
        var user = _context.Users.Local.SingleOrDefault(x => x.email_address == emailAddress);
        return user;
    }        
}

My UserControler:

[Route("api/Users")]
[ApiController]
public class UsersController : ControllerBase
{
    private readonly IUserService _userService;

    public UsersController(IUserService userService)
    {
        _userService = userService;
    }

    [HttpGet("GetAll")]
    public IActionResult GetAll()
    {
        var users = _userService.GetAll();
        return Ok(users);
    }

    [HttpGet("GetUser")]
    public IActionResult GetUser(string emailAddress)
    {
        var user = _userService.GetByUsername(emailAddress);
        return Ok(user);
    }
}

And GetAll() returns the whole graph for all Users. But GetUser() returns a single User with property Customer null.

How can I get the whole User graph including the property Customer?

Any help would be appreciated.

Microsoft Visual Studio Community 2019 - Version 16.10.3

Microsoft.AspNetCore (5.0.0)

Microsoft.EntityFrameworkCore (5.0.7)