Livedata observer doesnt work on first call

I have two fragments A and B , fragment A has an observer which is working fine. fragment A navigates to fragment B, which has a very similar observer that doesn't run on first call,.. I must navigate back to A then B again ! then after that it is working fine.. if i close the app and open it again same thing happens.

Fragment B :

private val viewModel: WorkOutViewModel by activityViewModels() {
    MainViewModelFactory(
        (requireActivity().application as WorkOutApp).repository
    )
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    Log.e("AdvanceFragment.kt", " opening advance fragment with ID ::: ${args.workoutId}   ")

    viewModel.populateAdvanceFragment(2) // this populates the advanceworkout list
    viewModel.advanceworkout.observe(viewLifecycleOwner, {
            if (it.size == 0) {
                Toast.makeText(requireContext(), "edit mode advance fragment ", Toast.LENGTH_LONG)
                    .show()
                return@observe
            }
    // populate ui 
    }
}

Data source is Room db query that returns a Flow<List> -- aslivedata()

ViewModel:

var advanceworkout: LiveData<List<WorkOutEntity>> = MutableLiveData<List<WorkOutEntity>>()

  fun populateAdvanceFragment(id: Int) { 
   
    viewModelScope.launch(Dispatchers.IO) {
        advanceworkout = repository.getWorkOut(id).asLiveData()
    }}

This fragment is called from another fragment using navigation graph :

Fragment A :

viewModel.simpleworkout.observe(viewLifecycleOwner, {
        if (it.size == 0) {
           
            Toast.makeText(requireContext(), "edit mode simple fragment ", Toast.LENGTH_LONG)
                .show()
            return@observe
        }
//populate ui
}

binding.advanceFragmentTv.setOnClickListener {
            val action =
                SimpleFragmentDirections.actionSimpleFragmentToAdvanceFragment(2)
            findNavController().navigate(action)
        }

First fragment opens from bottom navigation menu .. hosted in the single activity of this app .

How can I ensure that the observer runs when I navigate to the fragment ? why is it behaving like this anyway?