I’‘m not sure why this happens. I haven’‘t set up enough users to test, yet, but the place I’‘m sure I’'m seeing this – consistently – is with federated users. GTalk, specifically.
Things just aren’‘t quite working as I hear they should. There’'s no giving permissions at either end, when users are added from the other. No on-line status shows up, and no opportunity is granted to make it show up, at either end, no matter how or who is adding the contact. But when an apparently off-line contact is messaged, they may well be on-line. The act of talking to them does not, however, change their status in the main window.
I must be missing something obvious . . .