SerializationInfo's private Field m_nameToIndex gone in deployed WCF Service

I've got a serializable type that wraps Dictionary< string,object> which I'm using to exchange payload.

In its deserialization constructor, we were using reflection to get the (private) Field m_nameToIndex from the given SerializationInfo and used it to get the values stored in another (private) Field called m_data.

Obviously it contained the key as well as the index of the value within m_data

public PayloadDictionary(SerializationInfo info, StreamingContext context)
{
    _Dictionary = new Dictionary<string, object>();

    var SIType = typeof(SerializationInfo);

    var MI_nameToIndex = SIType.GetField("m_nameToIndex", BindingFlags.NonPublic | BindingFlags.Instance);
    var MI_data = SIType.GetField("m_data", BindingFlags.NonPublic | BindingFlags.Instance);

    Dictionary<string, int> nameToIndex = MI_nameToIndex.GetValue(info) as Dictionary<string, int>;
    object[] data = MI_data.GetValue(info) as object[];
    foreach (KeyValuePair<string, int> KV in nameToIndex)
    {
        var Object = data[KV.Value];
        if (Object != null)
            _Dictionary.Add(KV.Key, Object);
    }
}

When the service has been compiled and deployed to a Windows Server 2012 R2, I can't seem to get the field m_nameToIndex using BindingFlags.NonPublic | BindingFlags.Instance anymore.

Sadly this is some old code from another developer and I don't really know why this is done manually in the first place.