Why does EntityFramework populate my NotMapped field with a trash value - entity-framework

Question

Hello i am retrieving a IEnumerable<Something> using Entity Framework and this Something has an IEnumerable<SomethingElse> property that has the attribute [NotMapped] over it.
Why does entity framework populate this IEnumerable<SomethingElse> field with some unknown object that is neither null but can't be interogated with Linq throwing the following exception:
System.ArgumentNullException: 'Value cannot be null. (Parameter 'source')'
public class Something
{
[NotMapped]
public IEnumerable<SomethingElse> Items{get;set;}
}
public struct SomethingElse
{
//some fields
}
Insertion:
When i insert the list some elements have the NotMapped field null, while others not:
List<Something> toBeInserted=new List<Something>{ new Something{Items=null},new Something{Items=new[]{ new SomethingElse{}}};
//dbcontext insert the list....
Retrieval
List<Something>somethings=(await this.[SomeDBContext].Somethings).ToList();
Now at retrieval some elements have the NotMapped property null as it should be,while someothers have this weird non-null IEnumerable<SomethingElse>d_6that throws exception on any Linq interogation:
var weirdElement=somethings.First();
weirdElement.Items.Count(); //throws
weirdElement.Items.Any();throws
P.S I have checked this post regarding computed properties and it says: Materialize the list when you fetch it from the database to resolve the computed properties.That is what i have done.
I issue (await this.DBContext.Somethings).ToList() to no avail.
Some Something's still have their IEnumerable<SomethingElse> property not null.
Later Edit
This is a picture with what the
not mapped property of the element looks like:
I am computing the IEnumerable<SomethingElse> using an extension method:
public class Extension
{
public static IEnumerable<SomethingElse> Compute(this IEnumerable<SomethingSomethingElse> someOthers){}
}
The name of the type of the field IEnumerable<SomethingElse> is <Compute>d_6