According to XEP-0045 :

_The service MUST send all discussion history messages before delivering the room subject and any "live" messages sent after the user enters the room.

_

But from what i see on the PresenceModule code :

room.addOccupantByJid(senderJID, nickname, newRole);
...
...
...
sendHistoryToUser(room, senderJID, maxchars, maxstanzas, seconds, since, writer);

this leaves room for live messages to be sent to the user before it finishes delivering the full history.

I have been able to successfully generate this wrong behavior from the MUC.