ObservableCollection Not Updating WPF ListView

Question

I created an ObservableCollectionEx.cs class that inherits the ObservableCollection class to suppress notifications while the collection is being updated until it's done updating from the answer here.

The class:

public class ObservableCollectionEx<T> : ObservableCollection<T>
    {
        private bool _notificationSupressed = false;
        private bool _supressNotification = false;
        public bool SupressNotification
        {
            get
            {
                return _supressNotification;
            }
            set
            {
                _supressNotification = value;
                if (_supressNotification == false && _notificationSupressed)
                {
                    this.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
                    _notificationSupressed = false;
                }
            }
        }

        protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
        {
            if (SupressNotification)
            {
                _notificationSupressed = true;
                return;
            }
            base.OnCollectionChanged(e);
        }
    }

A collection of models is created in a class that is meant to update in response to a number of events. One is based on an observable sequence that simply updates the collection at an interval and another is based on a button click event. Stepping through the codes, I see that both events are causing the collection to update successfully, but only the button click causes the WPF ListView to be notified and updated accordingly. The UI is a WPF UserControl that is used to create a CustomTaskPane in Microsoft Word using VSTO.

The code that updates the collection via Observable sequence:

 public partial class CrossReferenceControl : UserControl, ICrossReferenceControl
    {
        private ICrossReferenceControlViewModel referenceControlViewModel;
        private IOpenDocumentModel OpenDocumentModel;
        private ICrossReferenceGuy CrossReferenceGuy;
        private bool isOpen;

        private IObservable<bool> openDocModelUpdateObservable;

        private static TimeSpan period = TimeSpan.FromSeconds(20);

        private IObservable<long> observable = Observable.Interval(period);

        public readonly Subject<bool> OpenDocModelUpdateActionSubject = new Subject<bool>();

        public ICrossReferenceControlViewModel ReferenceControlViewModel => referenceControlViewModel;
        public bool IsOpen 
        {
            get { return isOpen; }
            set { isOpen = value; }
        }
        public CrossReferenceControl(IOpenDocumentModel openDocumentModel, ICrossReferenceControlViewModel referenceControlViewModel, ICrossReferenceGuy crossReferenceGuy)
        {
            InitializeComponent();
            this.referenceControlViewModel = referenceControlViewModel;
            OpenDocumentModel = openDocumentModel;
            CrossReferenceGuy = crossReferenceGuy;
            //CrossReferenceControlViewModel controlViewModel = new CrossReferenceControlViewModel((OpenDocumentModel)openDocumentModel);
            DataContext = referenceControlViewModel;
            observable.Subscribe(O => OpenDocumentModel.UpdateCaptionsSubject.OnNext(IsOpen));
        }
}

The code that updates via button click event (this works fine):

        private void ButtonRefresh_Click(object sender, RoutedEventArgs e)
        {
            OpenDocumentModel.UpdateCaptionsSubject.OnNext(IsOpen);
        }

Note: The codes are cut down to provide only what I think is essential.