We have been discussing this here :
http://www.phpbb-seo.com/boards/advance ... t2770.html
It's a tricky one, but the behaviour is exactly the same as if you would use a natural link without a forum_id in the url : viewtopic.php?t=xx
It's the only case I know of where phpBB3 is not fully compatible with phpBB2 URLs.
Usually, when global announces are displayed in a forum, they use this forum id in link and everywhere, but if the id is not provided, like it's the case with our url rewriting, the script will first try the topic forum id, and since global announces do not have forum id, it ends up querying for the first postable forum.
The idea with not providing with the forum id to global announces was to prevent the global announces from being duplicates, since they are displayed in each forums and each time with a different forum id in url.
So it's indeed a tricky case, more trouble occurs when the lowest forum id (not cat) is not public, but this is another matter.
For the breadcrumb only, it's not a drama, but we'll think of a better way to handle these.
++