Why does my EF Core migration ignore the model's OnDelete settings?

I have the following code in an, what looks like a futile, attempt to configure the OnDelete behaviour for foreign keys:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    modelBuilder.Entity<Contact>()
        .HasOne(e => e.Gender)
        .WithMany()
        .HasForeignKey(e => e.GenderId)
        .OnDelete(DeleteBehavior.Restrict);

    modelBuilder.Entity<Contact>()
        .HasOne(e => e.Title)
        .WithMany()
        .HasForeignKey(e => e.TitleId)
        .OnDelete(DeleteBehavior.Restrict);
}

Yet when I generate the first migration, that creates this and another two tables, it creates constraints for Contact as follows:

constraints: table =>
{
    table.PrimaryKey("PK_Contact", x => x.Id);
    table.ForeignKey(
        name: "FK_Contact_Gender_GenderId",
        column: x => x.GenderId,
        principalTable: "Gender",
        principalColumn: "Id",
        onDelete: ReferentialAction.SetNull);
    table.ForeignKey(
        name: "FK_Contact_Title_TitleId",
        column: x => x.TitleId,
        principalTable: "Title",
        principalColumn: "Id",
        onDelete: ReferentialAction.Cascade);
});

Where the blazes does it get SetNull, on a non-nullable column, GenderId? Cascade is maybe a default, having ignored my configuration.