I have a very similar issue, after moving from galaxy s10e to iphone 12.
the messages are all messed up, and the Imessages is supper buggy.
first thing is if I get a message from someone you I received messages in the past- and I click on the notification, then the message I see is months old and not the right one, looks like it the last message I got from that sender before the switch.
to see the new message I need to close the Imessages and enter it once again- then its fine.
another symptom of this would be if I access a message from a sender that sent me messages in the past(before the switch to iphone- for example like the post office), and then switch to another app, or go to home for example, but not exiting the Imessage, and then reopen the Imessages then again I get the issue of seeing the last message from 6 months ago(before the switch to iphone).
and again if I exit the app completely and re enter - its fine.
one more symptom- i'm not sure is entirely related, when I get a message in the Imessages I get double notification- one in the moment of receiving the message and one exactly 2 minutes later.
this behavior happens only with Imessages, and not any other app.