Visual Studio 2017 and EF ignores setting propperty to null

I discovered an extremely strange behavior in Visual Studio 2017 in combination with Entity Framework I have the following instruction:

public ActionResult AngebotAufheben(int sachkostId)
{
    Sachkost sachkost = db.sachkosten.Find(sachkostId);
    sachkost.ausgewaehltesAngebot = null;
    if (sachkost.ausgewaehltesAngebot != null)
        sachkost.ausgewaehltesAngebot = null;
    db.Entry(sachkost).State = EntityState.Modified;
    db.SaveChanges();
    return RedirectToAction("Edit", new { id = sachkostId });
}

"ausgewaehltesAngebot" is of type "Angebot" , and both "Sachkost "and "Angebot" are managed through the Entity Framework.

The strange thing for me is that VS ignores

sachkost.ausgewaehltesAngebot = null;

and

if (sachkost.ausgewaehltesAngebot != null)

is not false.

If i set a breakpoint at

db.Entry(sachkost).State = EntityState.Modified;

VS tells me that sachkost.ausgewaehltesAngebot is of type System.Data.Entity.DaynamicProxies.Angebot_...

Is this a form of EF magic and intentional? I definitely do not understand it.

Many thanks for the help.

Update: Here is my Definition for Sachkost:

public class Sachkost
{
    public int SachkostId { get; set; }
    public string Name { get; set; }
    public KategorienSachkost kategorieNummer { get; set; }
    public decimal geplanteKosten { get; set; }
    public virtual Angebot ausgewaehltesAngebot { get; set; }
    public virtual List<Angebot> angebote { get; set; }
    public int massnahmeID { get; set; };
    [ForeignKey("massnahmeID")]
    public virtual Massnahme massnahme { get; set; }
    public string erstellerId { get; set; };
    [ForeignKey("erstellerId")]
    public virtual Mitarbeiter ersteller { get; set; }
}

And i am using EF6