You are correct, but, from our perspective, we say it in a different way.

The Server-Client communication protocol, in order to save bandwidth, forces unchanged fields not to be sent, but to be replaced by flags that identify unchanged fields.
We consider this as a not configurable implementation detail, as the client receives enough information to be able to retrieve the unchanged values from previous updates.
However, some of our client libraries lack support for this task and just forward the "unchanged" notification to application code, rather than resolve unchanged fields under the hood.
This is what I expect that happens in your case. Note that the "unchanged" notification is expressed differently depending on the kind of listener you are using.

Fortunately, this limitation is going to be removed with the forthcoming 3.5 release of Lightstreamer (announced in our July 2008 newsletter)
where a new listener type will be introduced for this purpose, for all client SDKs that are currently affected by the above limitation.