NGRX - Problem with testing effects that convert promises to observables with jasmine-marbles

I have a nrwl/nx workspace with a library called missions-shared This library has a feature state, under the key missions I also have a backend service, called BackendServiceClient. This backend returns promises (due to code generation)

My problem is with the fact that the backend returns promises, and jasmine-marbles being unable to recognize that:

I'm trying to mock the backend. I use a test provider for that

let backend = {
  get: () => Promise.resolve({items: []})
}

TestBed.configureTestingModule({
...
providers : [{
  provide: BackendServiceClient, useValue: backend
}]
...

This is the MissionsEffect :

@Injectable()
export class MissionsEffects {
  @Effect() loadMissions$ = this.dataPersistence.fetch(
    MissionsActionTypes.LoadMissions,
    {
      run: (action: LoadMissions, state: MissionsPartialState) => {
        return from(this.backend.get(new GetMissions())).pipe(
          map(r => new MissionsLoaded(r.items))
        );
      },

      onError: (action: LoadMissions, error) => {
        console.error('Error', error);
        return new MissionsLoadError(error);
      }
    }
  );

  constructor(
    private dataPersistence: DataPersistence<MissionsPartialState>,
    private backend: BackendClientService
  ) {}
}

Then I try to test these @Effect using jasmine-marbles

   actions = hot('-a-|', { a: new LoadMissions() });
   expect(effects.loadMissions$).toBeObservable(
     hot('-a-|', { a: new MissionsLoaded([]) })
   );

But I get this error in the tests :

  ● MissionsEffects › loadMissions$ › should work

    expect(received).toEqual(expected) // deep equality

    - Expected
    + Received

    - Array [
    -   Object {
    -     "frame": 10,
    -     "notification": Notification {
    -       "error": undefined,
    -       "hasValue": true,
    -       "kind": "N",
    -       "value": MissionsLoaded {
    -         "payload": Array [],
    -         "type": "[Missions] Missions Loaded",
    -       },
    -     },
    -   },
    -   Object {
    -     "frame": 30,
    -     "notification": Notification {
    -       "error": undefined,
    -       "hasValue": false,
    -       "kind": "C",
    -       "value": undefined,
    -     },
    -   },
    - ]
    + Array []

It seems like because I'm mocking the backend using a Promise (as it should), it doesn't recognize the returned Observable.

If I change the mock to :

let backend = {
  get: () => of({items: []})
}

Then the test succeeds.

This is the test :

describe('MissionsEffects', () => {
  let actions: Observable<any>;
  let effects: MissionsEffects;
  let backend = {
    get: () => of({items: []})
  }

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [
        NxModule.forRoot(),
        StoreModule.forRoot({}),
        EffectsModule.forRoot([]),
        RouterModule.forRoot([])
      ],
      providers: [
        MissionsEffects,
        DataPersistence,
        provideMockActions(() => actions),
        { provide: APP_BASE_HREF, useValue: '/' },
        { provide: BackendClientService, useValue: backend }
      ]
    });

    effects = TestBed.get(MissionsEffects);
  });

  describe('loadMissions$', () => {
    it('should work', () => {
      actions = hot('-a-|', { a: new LoadMissions() });
      expect(effects.loadMissions$).toBeObservable(
        hot('-a-|', { a: new MissionsLoaded([]) })
      );
    });
  });
});

I can confirm that the issue is not due to using @nrwl/DataPersistence, as the following @Effect produces the same error:

@Injectable()
export class MissionsEffects {

  @Effect() loadMissions$ = this.actions$
    .pipe(
      ofType(MissionsActionTypes.LoadMissions),
      switchMap(loadMission => {
        return from(this.backend.get(new GetMissions())
          .then(r => new MissionsLoaded(r.items))
          .catch(e => new MissionsLoadError(e)))
      })
  )

  constructor(
    private actions$: Actions,
    private backend: BackendClientService
  ) {}
}

Can someone help me understand exactly what is the problem here? Why can't I use mocked promises to test this?