Unable to update entity which is a 1 to 1 #8518

Hello I have an entity Xs it has a 1 to 1 relationship to XEffectiveness

[Table("Xs")]
public class X : FullAuditedEntity 
{
    public virtual int? Effectiveness { get; set; }
    
    [ForeignKey("Effectiveness")]
    public XEffectiveness EffectiveneFk { get; set; }
}

[Table("XEffectivenesses")]
public class XEffectiveness : FullAuditedEntity 
{
    public virtual int XId { get; set; }       
    public virtual X X { get; set; }
}

When I create a new record it successfully populates both tables correctly. However when I go to update the records it fails with Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while updating the entries. See the inner exception for details. ---> System.Data.SqlClient.SqlException: Cannot insert explicit value for identity column in table 'XEffectivenesses' when IDENTITY_INSERT is set to OFF.

My PortalDBContext.cs has the following:

modelBuilder.Entity<X>()
    .HasOne(x=> x.EffectiveneFk)
    .WithOne(u => u.X)
    .HasForeignKey<XEffectiveness>(b => b.XId);

To workaround the issue I have enacted the following:

private async Task<int> Update(CreateOrEditXDto input)
{
    var x = await _xRepository.FirstOrDefaultAsync((int)input.Id);
    ObjectMapper.Map(input, x);

    var TEMPeffectiveness = x.EffectiveneFk;
    x.EffectiveneFk = null;

    await _xRepository.UpdateAsync(x);

    await _lookup_xEffectivenessRepository.InsertOrUpdateAsync(TEMPeffectiveness);

    return x.Id;
}

Which essentially takes out the 1 to 1 of Effectiveness temporarily, saves the parent of the 1 to 1 which is X and then saves the Effectiveness.

I have asked also on stackoverflow and will update both if I get an answer.