Eager Loading using EF6 Fluent Api with a Composite Key Fails to Load Related Entity

Question

I must be creating the composite key incorrectly or connecting it to the related entity incorrectly. I can get it to work in a simplified scenario of a single key, but the composite key fails. I'll show both working and non working versions.

My classes:

public class Parts {
   public string PartID { get; set; }
   public string Revision { get; set; } 
}

public class InstanceHistories {
    public int HistoryID { get; set; }
    public string PartID { get; set; }
    public string Revision { get; set; }
    public virtual Parts Part { get; set; }
}

Fluent API - That works

 modelBuilder.Entity<Parts>().HasKey(p => new { p.PartID });

 modelBuilder.Entity<InstanceHistories>().HasKey(i => i.HistoryID);
 modelBuilder.Entity<InstanceHistories>().HasRequired(i => i.Part);

Repo call

history = await repo.All
                    .Include(b => b.Part)
                    .Where(i => i.PartID== partIDToSearch).ToListAsync();

This repo call will load back my related Part object without any issues.

When I change the Fluent API to make the part have a composite key, my repo call will no longer return the related entity.

Non working Fluent API update

modelBuilder.Entity<Parts>().HasKey(p => new { p.PartID, p.Revision });

I have tried a few different scenarios of .WithMany(), .HasForeignKey(), .HasOptional() but nothing really seems to kick the Part back. I even tried to Lazy Load and they wont return so I know something must be up with my setup. Has anyone come across this scenario?

Edit

Even when I force the SQL to be shown (ie uow.Context.Database.Log = Console.WriteLine;), I see the related entity appear in the joined result.

Also, I do not receive any errors. Just null return on the Part object.