hi
i tried something on my system to see what is happening and to verify yr results.
but could not force Messages to split conversations.
using iOS12 officially released version.
i had two Contacts in my address book for the same contact, but with different associated sending addresses.
sure enough, in Messages, the conversations yesterday were split.
this is not surprising.
but to try to force it into one Conversation, i closed Messages, put all addresses into one contact, and then deleted the other card.
i opened Messages and the two previous Conversations were unified into one Conversation.
i thought this was amazing, actually, since one of the threads previously is a thread that always got orphaned since the sender (a bank) seems to be using just regular SMS (i think). but the conversations became unified - as long as all that contact's "sending from" IDs/addresses were in its same unified Contact card on my mac/iPhone.
things to do:
- check to see if you have different contact cards for addresses then unify them
- use the new Sync button apple has put into Messages to sync across devices