The forum software is working as it should. NT means "no text" and once you put a period or other character in the "message body" box, it's not a "no text" post anymore.
I've just discovered that if I use the "old" version, the system overrides it. I.e. if I type (NT) and then put a dot in as text, the NT is eliminated from the header, presumably because the computer now recognises that it is not truly an NT message.
I infer therefore that one must let the computer determine that it is actually an NT message and leave it to do its thing (i.e. no dots in future).
BTW This is notwithstanding the Forum Feedback thread, but that thread is concerned with deliberately tricking the system methinks rather than trying to avoid inconvenience to others.