Observers don't get notified when LiveData is set through FileObserver

Question

I have a sharedViewModel with LiveData properties.

MainActivity:

    private val logViewModel by viewModels<LogViewModel>()

    fun startLocationUpdates() {
       val locationItem = LocationItem(...)
       logViewModel.addLogEntry(locationItem)
    }

    override fun onCreate(savedInstanceState: Bundle?) {

        val observer = object : FileObserver(applicationContext.filesDir.path + "/" + jsonFileName) {
            override fun onEvent(event: Int, file: String?) {
                if (event == FileObserver.MODIFY) {
                    loadLogViewModel()
                }
            }
        }
        observer.startWatching() 
    }

    fun loadLogViewModel() {
        val json = getData(this)
        val myType = object : TypeToken<ArrayList<LocationItem>>() {}.type
        if (json != null) {
            val listItems: ArrayList<LocationItem> = gson.fromJson(json, myType)
            for (item in listItems) {
                logViewModel.addLogEntry(item)
            }
        }
    }
    

LogFragment:

private val logViewModel: LogViewModel by activityViewModels()

logViewModel.locationListItems.observe(requireActivity()) {
    locationItemAdapter.updateLocations()
}

LogViewModel:

class LogViewModel : ViewModel() {
    var locationListItems: MutableLiveData<LocationItem> = MutableLiveData<LocationItem>()

    fun addLogEntry(locationItem: LocationItem) {
        locationListItems.postValue(locationItem)
    }
}

When I execute startLocationUpdates() it all works as expected. The observer gets notifications.

But when the logfile (json) is changed through the WorkManager (potentially within a different process/thread) and the FileObserver kicks in to load the JSON file and adds them to the LogViewModel, the Live data is not firing anything.

Why does it make a difference if the fileObserver is doing it, or if startLocationUpdates() would be doing it? I can only assume this is a threading issue. But I thought LivaData deals with that?