linq2db: [linq2db.Access] DC.update(object) doesn't update pointer to related tables

hello!

There’s no exception but during a simple update, the foreign key field is not updated. Here’s the code:

Dim myPrj As PrjMain
myPrj = DM.PrjMains.getByID(DC, myID).FirstOrDefault
myPrj.Title = tbxTitle.Text.Trim
myPrj.FkIDCompany = CType(ddlCustomer.SelectedValue, Integer)                'this one is not updated 
myPrj.FkIDCompanyBroker = CType(ddlBroker.SelectedValue, Integer)           'this one is OK!!!! 
myPrj.FkIDPrjType = CType(ddlProjectType.SelectedValue, Integer)
myPrj.FkUserNameConsultant = ddlConsulente.SelectedValue
DC.Update(myPrj)

Here is model extract:

[Table("PRJ-Main")]
public partial class PrjMain
{
[Column(),                              PrimaryKey,  Identity] public int       ID                          { get; set; } // Long
[Column("title"),                          Nullable          ] public string    Title                       { get; set; } // VarChar(255)
/// <summary>
/// Cliente
/// </summary>
[Column("FK_IDCompany"),                             Identity] public int       FkIDCompany                 { get; set; } // Long
/// <summary>
/// Segnalatore
/// </summary>
[Column("FK_IDCompanyBroker"),             Nullable          ] public int?      FkIDCompanyBroker           { get; set; } // Long
/// <summary>
/// Importo da riconoscere al segnalatore
/// </summary>
[Column("amountFeeBroker"),                Nullable          ] public decimal?  AmountFeeBroker             { get; set; } // Currency
/// <summary>
/// Link a tabella dei tipi di progetto
/// </summary>
[Column("FK_IDPrjType"),                             Identity] public int       FkIDPrjType                 { get; set; } // Long
/// <summary>
/// Consulente
/// </summary>
[Column("FK_UserNameConsultant"),       NotNull              ] public string    FkUserNameConsultant        { get; set; } // VarChar(50)
/// <summary>
/// Data inserimento a sistema
/// </summary>

FkIDCompany is not updated when FK_IDCompanyBroker is correctly done. The difference is that FkIDCompany is mandatory (type integer) instead of FK_IDCompanyBroker (type nullable(of integer))

If I change FkIDCompany as not mandatory (nullable(of integer)) all work good!

is there a problem?

Thank you!

Environment details

linq2db version: 3.4.2 Database Server: MS Access Database Provider:
Operating system: Windows 10 .NET Framework: 4.7.2