Why doesn't the data persist when I use LiveData in the ViewModel?

I am switching the screen using the Navigation Component.

In the fragment screen of the bottom A menu, i can add a recyclerview item dynamically through the button.

If i press the button on this screen, it moves to another fragment where data can be selected.

If i select data on the converted fragment screen, it returns to the previous screen and adds a recycler view item based on the selected data.

This process is repeated.

However, even if I repeat this process, the item is not dynamically added.

I add items to the List of LiveData by using the ViewModel, but as a result of debugging, the size of the list type of LiveData does not increase from only one.

At least as far as I know the data should be persisted because using viewmodel is not affected by lifecycle.

But the problem I have is that it seems to be initialized and saved every time because of the screen change.

Why is this?

ViewModel

class WriteRoutineViewModel : ViewModel() {
    private var _items: MutableLiveData<ArrayList<RoutineModel>> = MutableLiveData(arrayListOf())
    val items: LiveData<ArrayList<RoutineModel>> = _items

    fun addRoutine(workout: String) {
        val item = RoutineModel(workout, "TEST")

        item.setSubItemList(detailItem)
        _items.value?.add(item)
        _items.value = _items.value
    }
}

Fragment

class WriteRoutineFragment : Fragment() {
    private var _binding : FragmentWriteRoutineBinding? = null
    private val binding get() = _binding!!
    private lateinit var adapter : RoutineAdapter
    private val args : WriteRoutineFragmentArgs by navArgs()
    private val vm : WriteRoutineViewModel by viewModels { WriteRoutineViewModelFactory() }

    override fun onCreateView(inflater: LayoutInflater,
                              container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        _binding = FragmentWriteRoutineBinding.inflate(inflater, container, false)

        adapter = RoutineAdapter(::addDetail, ::deleteDetail)
        binding.rv.adapter = this.adapter
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        args.workout?.let { workout ->
            vm.addRoutine(workout)
            Toast.makeText(context, workout, Toast.LENGTH_SHORT).show()
        }

        vm.items.observe(viewLifecycleOwner) { updatedItems ->
            adapter.setItems(updatedItems)
        }
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
}

UPDATE nav_graph

<fragment
        android:id="@+id/writeRoutine"
        android:name="com.example.lightweight.fragment.WriteRoutineFragment"
        android:label="fragment_write_routine"
        tools:layout="@layout/fragment_write_routine" >
        <action
            android:id="@+id/action_writeRoutineFragment_to_workoutListTabFragment"
            app:destination="@id/workoutListTabFragment" />
        <argument
            android:name="workout"
            app:argType="string"
            app:nullable="true"
            android:defaultValue="@null"/>
    </fragment>