Mockito mock not returning paging/flow data

Question

I am running into an issue trying to mock paging data coming from a repository. Here is the portion in my viewmodel I would like to test:

   // watch for filter to change, on charge reset the paging query
   val items = filterParameter.switchMap { filter ->
       Pager(
          config = PagingConfig(pageSize = 20),
          initialKey = null,
          pagingSourceFactory = {
             repository.observeItems(filter)
          }
       ).liveData
    }.asFlow().map { pagingData ->
       pagingData
          .map {
             ListItem(it)
          }
          .insertSeparators { item1: ListItem?, item2: ListItem? ->
             // logic I want to test
          }
    }.cachedIn(viewModelScope)

And my test:

   @Test
   fun testTransformation() = runTest {

      val repository = mock<MyRepository>()
      `when`(repository.observeItems(Mockito.any()))
         .thenAnswer {
            val flow = flowOf(
               listOf(
                  Item(
                     id = "1",
                     date = Date()
                  )
               )
            )

            val pagingSourceFactory = flow.asPagingSourceFactory(coroutineScope = this)
            pagingSourceFactory()
         }

      val viewModel = MyViewModel(
         repository = repository,
      )

      val items: Flow<PagingData<ListItem>> = viewModel.items
      val snapshot: List<ListItem> = items.asSnapshot() {}

      Assert.assertEquals(1, snapshot.size) // fails as zero items are in the snapshot
   }

I have verified that when running the test, the pager is setup in the viewmodel, and the mock flow is called. However I am not seeing the mocked data come back. Any ideas?

Demo android project on GitHub here: https://github.com/newmanw/android-paging-test