Nest JS two instances of the same provider

Hi on a test suite it appears to me that I have 2 living instances of a same provider, one for the implementation and another one for the real implementation.

I base my conclusion in a fact that on my test I tried replace a method by a jest.fn call but still, on the service I am testing the method still points to the original implementation.

What makes it even more odd is that I was able to mock another service performing exactly the same procedure, as if, depending on how those services were injected (where they came from in the container graph) it would or not work.

I'll try to share some snippets, but of course, only a small repo could actually reproduce it, but perhaps someone has an insight:

 beforeAll(async done => {
    app = await Test.createTestingModule({
      imports: [
        SOME_MODULES,
      ],
    providers: [
      EssayApplicationService,
      ReviewFacade,
      ExamCacheResultService,
    ]

    essayApplicationService = app.get<EssayApplicationService>(EssayApplicationService)
    reviewFacade = app.get<ReviewFacade>(ReviewFacade)
    examCacheResult = app.get<ExamCacheResultService>(ExamCacheResultService)
    await app.init()
    done()
  })

it('should invoke review only once', async done => {

    reviewFacade.startReview = jest.fn() --> this works
    examCacheResult.clearCachedResult = jest.fn() --> this fails

    await essayApplicationService.finishApplication()

    expect(reviewFacade.startReview).toHaveBeenCalledTimes(1)
    expect(reviewFacade.startReview).toHaveBeenCalledWith(expect.objectContaining({ id: 1 }))
    expect(examCacheResult.clearCachedResult).toHaveBeenCalledTimes(1) ---> here this fails, although it's called!!

So,the issue boils down to the fact that I'm 100% positive that both methods were called on the service under test, but the second for some reason wasn't replaced by the mock