SIgning in as Inivisble status causes no presesnce updates.
When change your status directly from 'Offline' to 'Invisible', Gajim recieves the current status of all contacts, but it does not recieve any updates from contacts. So if someone signs in after you sign in you will not be notified. As soon as you change to the 'Available' status, Gajim recieves all the updates of presences.