Entity Framework Core 2.0: One-To-Many-Relationship only works once

I'm using ASP.NET Core 2.0 with Angular-Frontend and Entity Framework Core 2.0. I have following DbSets: DbSet<Vocabulary> Vocabularies and DbSet<Word> Words, while the class Vocabulary contains a collection of words: List<Word> Words { get; set; }. In my API-Controller I've written a method which provides the vocabularies to the frontend as a response to an HTTP-Get-request. To test my database-connection and everything, I used the same method to create an initial record into my database:

[HttpGet]
public IEnumerable<Vocabulary> Get()
{
    if (!Context.Vocabularies.Any())
    {
        Context.Vocabularies.Add(new Vocabulary
        {
            Name = "MyVocabulary",
            Words = new List<Word>
            {
                new Word
                {
                    PrimaryLanguage="Test",
                    SecondaryLanguage="Test",
                    Score= 0
                }
            }
        });
        Context.SaveChanges();
    }
    return Context.Vocabularies;
}

Now, the weird thing is that when the DB is empty, everything works as expected. When I reload the page (or restart the local IIS) and the HTTP-GET-request happens again, I get all vocabularies with correct ID's etc. but the property Words is null... I'm using a local SQL Server database. The tables seem ok, the words have the correct VocabularyId.

I just found out that there's a method called Include(). Tried it the following way: return Context.Vocabularies.Include(v => v.Words);, but didn't work either.

Vocabulary.cs:

public class Vocabulary
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual List<Word> Words { get; set; }
}

Word.cs:

public class Word
{     
    public int Id { get; set; }
    public string PrimaryLanguage { get; set; }
    public string SecondaryLanguage { get; set; }
    public double Score { get; set; }
}

Any ideas?