LiveData onResume not fetching new value, on pre Android 7 SDKs

Question

I am using LiveData with Activity. Everything is working when I test on Android 7, but has issues when I test on lower Android SDK versions.

1. When I close app, send some messages, then open app again, I am fetching received values from ViewModel via LiveData.
2. But if I open new activity over current one, receive some messages in ViewModel, and return back to that activity I am not getting those messages in Activity.

In first case onPause, onStop then onStart, onResume is called. In second case onPause then onResume is called. Also in second case I have active observer, but I am not getting received messages.

ChatViewModel

class ChatViewModel : ViewModel() {

    var adapterMessagesLive: MutableLiveData<AdapterChatItems> = MutableLiveData()

fun addMessage() {
   adapterMessagesLive.value = AdapterChatItems(items, addDirection)
}

}

ChatActivity

public class ChatActivity implements LifecycleRegistryOwner {

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

    viewModel = ViewModelProviders.of(this).get(ChatViewModel.class);
    viewModel.getAdapterMessagesLive().observe(this, adapterChatItemsObserver -> {
               getAdapter().addMessages(adapterChatItemsObserver);
    }
});
}

Update

After more debugging, I found out that in Android 7 activity continue receive values from LiveData after onPause is called. But on older Android SDK its not working.