Entity Framework Core v6.0.5 foreign key properties returning unexpected null

Question

I read a LoginUser (an extension of IdentityUser) from the AspNetUsers table with Entity Framework Core v6.0.5.

When I attempt to read its foreign key property Company, the property is always null - why?

i.e.

var user = dbContext.Users
                    .FirstOrDefault(...);// A non-null LoginUser is returned

var companyId = user.CompanyId; // results in Integer 1 - as expected

var company = user.Company; // company is always null

Table definitions:

public class LoginUser : IdentityUser
{
    public int CompanyId { get; set; } = -1;
    public virtual Company Company { get; set; }
}

public class Company
{
    public int Id { get; set; } = 0;
    public virtual ICollection<LoginUser> LoginUsers { get; set; }
    [Required]
    [MaxLength(128)]
    public string TradingName { get; set; } = string.Empty;
}

A foreign key index exists on the LoginUser table generated for the column CompanyId.

Any ideas what I have omitted to do?

AspNetUsers table data:

Id                                      CompanyId   Other User fields not shown...
8c568150-ecf8-46f4-8ec1-db9a7a66daaa    1   

Companies table data:

Id  TradingName 
-----------------------
-1  Unassigned Company
1   Discovery