EntityFramework does not update some fields

I have a problem with EntityFramework-6, I am trying to create a "disconnected" entity to simply save the changes of the fields that I want.

My context (DataContext.cs) has a member named Demos, and that entity type (Demo.cs) is derived from a class called ModelBase.

In my database, my "Demo" table is created as such as I have it in the DataContent.

When saving the changes, EF, only saves the changes to the Demo.cs class and not those inherited from ModelBase.cs.

This is the code that fails:

using (var contexto = new DataContext())
{
    // Creo la entidad.
    var demo = new Demo { Id = 1 };

    // Lo agrego al control de cambios.
    context.Demos.Attach(demo);

    // Este cambio si lo actualiza.
    obj.Nombre = "Carlos";

    // Este cambio NO LO ACTUALIZA.
    obj.Modified = DateTime.Now;

    context.SaveChanges();
}

This other example does work:

using (var contexto = new DataContext())
{
    // Creo la entidad.
    var demo = new Demo { Id = 1 };

    // Lo agrego al control de cambios.
    context.Demos.Attach(demo);

    // Este cambio si lo actualiza.
    demo.Nombre = "Carlos";

    // Este cambio si lo actualiza si lo especifico explicitamente.
    demo.Modified = DateTime.Now;
    context.Entry(demo).Property(x => x.Modified).IsModified = true;

    context.SaveChanges();
}

DataContext

public class DataContext : DbContext
{
    public DbSet<Models.Demo> Demos { get; set; }
}

Demo.cs

public class Demo : ModelBase
{
    public string Nombre { get; set; }

    public string Apellidos { get; set; }
}

ModelBase.cs

public class ModelBase : Core.DataAccessLayer.IModelBase
{
    /// <summary>
    /// Crea una nueva instancia de <see cref="ModelBase"/>.
    /// </summary>
    public ModelBase()
    {
        this.Created = DateTime.Now;
        this.Modified = DateTime.Now;
    }

    /// <summary>
    /// Obtiene o establece el identificador de un registro.
    /// </summary>
    [Key]
    public int Id { get; set; }

    /// <summary>
    /// Obtiene o establece la fecha de creación.
    /// </summary>
    public DateTime Created { get; set; }

    /// <summary>
    /// Obtiene o establece la fecha de modificación.
    /// </summary>
    public DateTime Modified { get; set; }
}

Edit:

I marked the ModelBase fields as "virtual", and now if you update it, however, it is not constant.

In some cases, execute this query:

exec sp_executesql N'UPDATE [dbo].[Demoes]
SET [Nombres] = @0, [Modified] = @1
WHERE ([Id] = @2)
',N'@0 nvarchar(max) ,@1 datetime2(7),@2 int',@0=N'25/05/2016 9:02:18',@1='2016-05-25 09:02:18.7027657',@2=1

And on other occasions this:

exec sp_executesql N'UPDATE [dbo].[Demoes]
SET [Nombres] = @0
WHERE ([Id] = @1)
',N'@0 nvarchar(max) ,@1 int',@0=N'25/05/2016 9:02:14',@1=1