If you have enabled group chat history before and have messages saved in the history. Disabling the group chat history should not show chat history to users when they join the chat as described in that admin page. Instead, it shows the history saved before the chat history was disabled but never overwrites it with new messages. I suppose the chat history needs to be checked and cleared when disabling.
This has been tested on and affects Openfire 3.5.2 running on both Linux and Windows.