In some circumstances fields may be omitted from re-sent messages.

We tracked this down to the following line in Session.parseMessage(String):
msg.fromString(messageData, dataDictionary, false);
In our case there was a repeating group used in outbound messages only that was not present in the data dictionary. Those fields silently disappeared.

So, a workaround is to edit the data dictionary. But given the silent misbehaviour, I would rate this a bug.

Probably could fix this by creating a subclass of Message that treats the body as an immutable string --> overrides to parseBody, calculateString, calculateLength and calculateTotal. Hopefully there is a more elegant solution.