could show up later
this post should be listed in this channel, but it is not. The post is 1 month old and has never been edited
this post is listed in this channel, but only in the initial version. The post has been edited 7 days ago
The !something doesn't do auto-completion with existing forum channel names.
I always use the @something+ tag (sometimes even without the + because that's what the auto-completing interface proposes me when I start typing @)