Please post your java object code.
Also what versions are you running, etc etc?
Also please post your code that sends and receives the objects and checks the timestamp.
Sorry, I've just found out what the issue was.... my own stupid mistake!
The second process that sends the first message was actually also receiving it back before the first process had set the timestamp, so naturally it would be null. It was the fact that the second process was reporting to have received the message back before the first process had updated the timestamp that gave it away.