Knockout viewmodel mapping stops subscription firing

I have following master viewmodel

var masterVM = (function() {
  this.PatientViewModel = ko.mapping.fromJS(new PatientViewModel());
  this.MedicalBenefit = new MedicalBenefitViewModel(this.PatientViewModel);
})();

and following view models

var PatientViewModel = function() {
  var self = this;

  self.FirstName = ko.observable();
  self.LastName = ko.observable();
  self.NationalHealthFundId = ko.mapping.fromJS(null);

  self.NationalHealthFundId.subscribe(function(newValue) {
    alert("subscribe from patient");
  });
};

var MedicalBenefitViewModel = function (patient) {
    var self = this;

    self.patient = patient;
};

and somewhere in custom binding (bind to patient):

...

var observable = valueAccessor();

ko.mapping.fromJS(patient, {}, observable);

...

where patient is simple plain json object representing patient.

After custom binding execution, the subscription ( alert("subscribe from patient"); ) stops working, why ?

You have set self.NationalHealthFundId = ko.mapping.fromJS(null);. You can only subscribe to an observable. For example, this works:

var PatientViewModel = function() {
  var self = this;

  self.FirstName = ko.observable();
  self.LastName = ko.observable();
  self.NationalHealthFundId = ko.observable();

  self.NationalHealthFundId.subscribe(function(newValue) {
    alert("subscribe from patient");
  });
};

var vm = new PatientViewModel()
ko.applyBindings(vm);

vm.NationalHealthFundId(1);
vm.NationalHealthFundId(2);

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