Why doesn't Word's undo record work with certain Actions?

Question

I've created a very simple class that should interface with Microsoft Word's UndoRecord in a VSTO addin.

public class ChangesTracker : IDisposable {
    private Word.UndoRecord undoRecord;

    public ChangesTracker(string name) {
        Globals.ThisAddIn.Application.ScreenUpdating = false;
        undoRecord = Globals.ThisAddIn.Application.UndoRecord;
        undoRecord.StartCustomRecord(name);
    }

    protected virtual void Dispose(bool disposing) {
        if (disposing) {
            if (undoRecord != null) {
                undoRecord.EndCustomRecord( );
                Globals.ThisAddIn.Application.ScreenUpdating = true;
                undoRecord = null;
            }
        }
    }

    public void Dispose() {
        Dispose(true);
    }

    ~ChangesTracker() {
        Dispose(false);
    }
}

I use it with a simple using statement around all the operations that should be grouped

e.g.:

using (var tracker = new ChangesTracker("Something")) {
    /// code
}

This seems to be working the majority of the time, except for anything that gets the selection's XML. For example this breaks it.

var xml = Globals.ThisAddIn.Application.Selection.Range.WordOpenXML;
selectionRoot = new XmlDocument( );
selectionRoot.LoadXml(xml);

When inspecting the UndoRecord, the property IsRecordingCustomRecord changes to false and the UndoRecord is ignored from then on. Am I doing something wrong? Is this perhaps yet another bug in Word and if so how do I go around it?