There’‘s a bug that occurs when a user that’'s logged in more than once (iow using multiple resources) and subscribes to the presence of another user.
At the end of the subscription process, the initiating user receives a presence subscribed stanza at all his resources (so far, so good). Sadly, the initial presence of the other users is routed to only one resource.
Browsing the code, I suspect lines 153 through 156 of the org.jivesoftware.wildfire.handler.PresenceSubscribeHandler.java class (revision 4113) to be responsible: as far as I traced into the code, only one presence packet is sent by those lines.
There’‘s no obscuring this bug by assuming a default ‘‘online’’ presence to users of which a ‘‘subscribed’’ stanza is received: if the other side agreed to the subscription while you were offline and dropped offline after that, you’‘d see an ‘‘online’’ user that’'s in reality not. (Besides that, users could actually be away or dnd without you noticing it).