Why can't I add an entity with children on a new context via EF Core?

Question

According to How to add a parent record with its children records in EF Core the following should work:

Model Builder

mb.Entity<IdentifierTable>().HasKey("StockID");
mb.Entity<StockTable>().HasKey("StockID");
mb.Entity<ProductTable>().HasKey("IdentifierID");
mb.Entity<GroupsTable>().HasKey("GroupId");

mb.Entity<StockTable>()
    .HasOne(x => x.Identifier)
    .WithOne(y => y.Stock)
    .HasForeignKey<IdentifierTable>(y => y.StockID);

mb.Entity<StockTable>()
    .HasOne(x => x.Group)
    .WithMany(y => y.Stock)
    .HasForeignKey(y => y.GroupId);

mb.Entity<ProductTable>()
    .HasOne<IdentifierTable>(x => x.Identifier)
    .WithOne(y => y.Product)
    .HasForeignKey<IdentifierTable>(y => y.IdentifierID);

base.OnModelCreating(mb);

Relevant parts of StockTable

public int StockID { get; set; }
public int? GroupId { get; set; }
[...]
public IdentifierTable Identifier { get; set; }
public GroupsTable Group { get; set; }

Relevant parts of IdentifierTable

public int IdentifierID { get; set; }
public ProductTable Product { get; set; }
public int StockID { get; set; }
public StockTable Stock { get; set; }

Relevant parts of ProductTable

public int? IdentifierID { get; set; }
public IdentifierTable Identifier { get; set; }
[...]

Relevant parts of GroupsTable

public int GroupId { get; set; }
public List<StockTable> Stock { get; set; }

The actual program

foreach (var item in contexts.originalContext.StockTable
                     .Include(x=>x.Group)
                     .Include(x=>x.Identifier)
                     .ThenInclude(y=>y.Product))
{
        contexts.destinationContext.StockTable.Add(item);
        contexts.destinationContext.SaveChanges();
}

However, when saving the changes, I get an exception:

violation of FOREIGN KEY constraint "FK_EST_IDENT_PROD" on table "ProductTable" Foreign key reference target does not exist

I understood EF Core should add StockTable's child properties as well (IdentifierTable, ProductTable, and distinct GroupsTable). What am I doing wrong here?

The reason I'm using FluentAPI is because I'm working with a pre-made database, I can't edit table/column names, nor relationships. Could it be because IdentifierTable.StockID is both primary key and foreign key?

I think I might have found a clue: Checking a tentative migration, I found EF Core is creating two foreign keys alright. However, it is creating both on IdentifierTable. The relationships I aim for are:

• One StockTable has one and only one IdentifierTable.
• One IdentifierTable has one or none ProductTable.
• One StockTable has one or none GroupsTable.
• One GroupsTable has none, one, or multiple StockTable.

I think I might not be able to use this database with EF Core. I tried scaffolding the models database-first, and EF Core didn't manage to understand a single foreign key relation.