Cypress test using callFake from "sinon" does not return observable

[Philip-Grozdanov]

I am running cypress test and I am stubbing(using callFake sinon function) by creating a new object which is shown as observable(when logging), but once returned in the cypress test from the mocked class it is not . Here is the main class :

export class EmployeeScheduleModel implements IEmployeeScheduleModel {
 private _employeeNeedsNotification = false;
 public debugTag = "";
 constructor(
   private _start: Date,
   private _end: Date,
   private _employee: EmployeeModel,
   private _shifts: IModelCollection<IEmployeeShiftModel>,
   private _absences: EmployeeAbsenceModel[] = [],
   private _holidays: HolidayModel[] = [],
   private _placeholders: PlaceholderModel[] = []
 ) {
   _shifts.getAll().forEach((shift) => shift.attachTo(this, false));
   makeAutoObservable(this);
   reaction(
     () => this._shifts.getAll().map((shift) => this.anyShiftDataChanged(shift)),
     () => (this._employeeNeedsNotification = true)
   );
   this._holidays = _employee?.holidays;
 }

I have created a class to stub me the return value of getEmployeeSchedules of the persistance which retrieves the schedules :

export class DefaultEmployeeSchedulePersistence {
  private readonly _mockEmployeePersistence: IEmployeeSchedulePersistence;
  private _getEmployeeSchedulesStub: Cypress.Agent<sinon.SinonStub>;

  constructor() {
    this._mockEmployeePersistence = this.create();
  }
  private create(): IEmployeeSchedulePersistence {
    const mock: Partial<IEmployeeSchedulePersistence> = {};
    this._getEmployeeSchedulesStub = cy.stub();
    mock.getEmployeeSchedules = this._getEmployeeSchedulesStub;
    this._getEmployeeSchedulesStub.callsFake(
      (employee: EmployeeModel, startArg: Date, endArg: Date) => {
        const schedule = new EmployeeScheduleModel(
          startArg,
          endArg,
          employee,
          ModelCollection.empty()
        );
        console.log("SET_UP_IS_OBSERVABLE", isObservable(schedule)); //returns true;
        return [schedule];
      }
    );
    mock.saveEmployeeSchedules = cy.stub().resolves();

    return mock as IEmployeeSchedulePersistence;
  }

  get mockEmployeePersistence(): IEmployeeSchedulePersistence {
    return this._mockEmployeePersistence;
  }
}

but when the cypress test runs it calls the stubbed class like this :

persistence.employeeSchedulePersistence.getEmployeeSchedules(
      this.employee,
      this.from,
      this.to
    ); 

but none of the objects is observable by logging with isObservable(obj) method and the content is the same I have mocked in the DefaultEmployeeSchedulePersistence class.

Why does when I set up objects with sinon stub before returning them they are observable but once they are fetched in the cypress test the returned result is not observable?

It seems like if I callFake and return objects which are observable , where they will be returned they will lose their observability