In article ,
Dave Plowman (News) wrote:

Of course. I wasn't replying to any particular post so just replied to the
subject. If a newsreader can't cope with that it's badly designed.

As in started a new post and just typed the Subject yourself? That would
explain it. The "correct" answer would have been for the reader to display
it as a new thread that just happened to have the same subject. Many (most?)
client these days just thread on subject and largely ignore the headers :-(

If you hit reply to any post then it should have added a header to show
which article you were referencing - many clients don't these days :-(

Darren