EF property set accessor never called

I have recently been working on migrating a site to using .Net4.5.2 with an APIController. I have a Profile Entity which has the following 2 properties that allow me to save a string to a mysql database, whilst allowing me to implement / manipulate it as a List<int>.

[System.ComponentModel.DataAnnotations.Schema.NotMapped]
public List <int> Subscriptions {
    get 
    {
      if (_Subscriptions == null)
        return new List < int > ();

      string[] ts = _Subscriptions.Split(',');
      return ts.Select(int.Parse).ToList();
    }
    set 
    {
      _Subscriptions = String.Join(",", value.ToArray());
    }
}

[EditorBrowsable(EditorBrowsableState.Never)]
public string _Subscriptions { get; set;}

// For test purposes
public List<int> Temp { get; set; }

I am sending an array of int to the APIController: public Profile CreateProfile(Profile profile). I have populated both the Profile.Temp and Profile.Subscriptions properties with [12345]. When it hits the breakpoint within CreateProfile, I can see that Temp has correctly populated with the value whilst Subscriptions has no value assigned. Additionally the breakpoint within the Subscriptions set accessor is never hit.

How can I make the set accessor run for the Subscriptions?

Note that if I declare Subscription as public List<int> Subscriptions { get; set; } this works identically to Temp (so I know that the data being sent is correct).