Bug tracker
Moving Global announcement no longer appears 'globally' (fix completed in vcs)
It involves the use of the MCP.
Moving a Global Announcement from one forum to another forum disables it's Global Announcement capabilities.
Although it is still set to topic_type = 3
The announcement will only display in the forum you move it to, (unless you selected to display the "shadow" topic)
Comments / History
Also, the other way to do it would be to go to the forum you only want it to appear in, and change it to announcement.
So... not a bug?