Topics Assignment: incorrectly manage mail related to existing topic but having empty reply-to-id and topic id (and non-empty refs)

In that particular case, most useful fields are empty (thread-id, reply-to-id...), subject equals subject of an existing topic, and references have not been "seen" before (even if not empty).
Usually we would have considered that this topic was "new" and was to be created. The fact that references are not empty seem to indicate that this is not the case though (it is "linked" to other mails).
So we should suppose that this mail relates in fact to the existing topic (the one with exactly same subject), and that somehow the refs have not been seen because either related relation has not been loaded yet, or was "off" the mailing-list for some time then back into again.