Vista - microsoft.public.windowsxp.general posts reappear after "catch up"

I have noticed that after using the "catch up" option in
microsoft.public.windowsxp.general to mark all posts as read and then
closing WLM, on reopening WLM all the posts that were supposed to be
marked "read" appear again, even though I have view set to "hide read
messages".

These are not newly downloaded posts but the same ones. I have checked
to see if there is "X messages not downloaded" in View Applied (bottom
left) just to make sure they are not "similar" posts.

At first I thought that I had changed the view to "show all messages"
but all posts are in bold (i.e. unread).

I am subscribed to 10 newsgroups (both MS and BT) but this only happens
in the XP newsgroup.

Anybody else noticed this?