EF Core - composite primary key removes one index

I have this entity:

[Table("order")]
public class Order
{
    [Key]
    [Column("id")]
    public Guid Id { get; set; }

    [MaxLength(128)]
    [Column("description")]
    public string Description { get; set; }

    [ForeignKey("Student")]
    [Column("student_id")]
    public Guid StudentId { get; set; }

    [ForeignKey("Employer")]
    [Column("employer_id")]
    public Guid EmployerId { get; set; }

    [ForeignKey("Customer")]
    [Column("customer_id")]
    public Guid CustomerId { get; set; }

    public virtual Guid Student { get; set; }
    public virtual Guid Employer { get; set; }
    public virtual Guid Customer { get; set; }
}

And I see next snapshot of DbContext:

modelBuilder.Entity("DataInterface.Models.Order", b =>
{
    b.Property<Guid>("Id")
        .ValueGeneratedOnAdd()
        .HasColumnName("id");

    b.Property<string>("Description")
        .HasColumnName("description")
        .HasMaxLength(128);

    b.Property<Guid>("StudentId")
        .HasColumnName("student_id");

    b.Property<Guid>("EmployerId")
        .HasColumnName("employer_id");

    b.Property<Guid>("CustomerId")
        .HasColumnName("customer_id");

    b.HasKey("Id");

    b.HasIndex("StudentId");

    b.HasIndex("EmployerId");

    b.HasIndex("CustomerId");

    b.ToTable("order");
});

How can you see - all fine. But I need to create a composite key and delete primary key. So, let's do it. My new code of the entity (without Id primary key):

[Table("order")]
public class Order
{
    [MaxLength(128)]
    [Column("description")]
    public string Description { get; set; }

    [ForeignKey("Student")]
    [Column("student_id")]
    public Guid StudentId { get; set; }

    [ForeignKey("Employer")]
    [Column("employer_id")]
    public Guid EmployerId { get; set; }

    [ForeignKey("Customer")]
    [Column("customer_id")]
    public Guid CustomerId { get; set; }

    public virtual Guid Student { get; set; }
    public virtual Guid Employer { get; set; }
    public virtual Guid Customer { get; set; }
}

I add new behavior to the OnModelCreating method of my context:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Order>()
        .HasKey(x => new { x.StudentId, x.EmployerId, x.CustomerId });
}

When I add new migration then I see that the index of StudentId was deleted. And I don't understand why? My new snapshot of DbContext:

modelBuilder.Entity("DataInterface.Models.Order", b =>
{
    b.Property<Guid>("Id")
        .ValueGeneratedOnAdd()
        .HasColumnName("id");

    b.Property<string>("Description")
        .HasColumnName("description")
        .HasMaxLength(128);

    b.Property<Guid>("StudentId")
        .HasColumnName("student_id");

    b.Property<Guid>("EmployerId")
        .HasColumnName("employer_id");

    b.Property<Guid>("CustomerId")
        .HasColumnName("customer_id");

    b.HasKey("StudentId", "EmployerId", "CustomerId");

    b.HasIndex("EmployerId");

    b.HasIndex("CustomerId");

    b.ToTable("order");
});

Why one of three indexes was deleted? Why b.HasIndex("StudentId"); line was deleted? What's wrong?

Note: I don't see this problem with a composite key from two fields in another tables.