Next, review the messaging & account settings on each respective iOS device. Make sure each is up to date too.
If that doesn't solve your issue, time to hit the closest Apple genius bar.
Both my wife and I have been experiencing this problem at the same time. Our accounts are not linked in any way. Text message threads are being merged for no reason. Let me explain. I will call myself A, my wife B, and our friends C and D. We had a group text message thread going with A,B, and C. Then D sent a group message to A and B (nothing to do with C). However, D's message appeared in the first thread with A, B, and C for no reason. And we each got a different notification on our messages. A's said that D added D to the conversation (which should be impossible). B's said that a phone number added D to the conversation. And D's said that A added C to the conversation. This is very bizarre and also a privacy risk since random people's messages are being added to entirely separate threads. This kind of thing has happened a few times. What could be going on?