So my wife and I and my mother in-law all share one Apple ID. We have for many years, it's never been an issue. We each have our own email and phone number associated with the Apple ID. Since I've moved to iOS 12, any messages I receive from the two of them (both on 11.4.1) get combined into one thread. In my list of messages, whichever one of them messaged me most recently is the person that is shown as the author of the whole thread, and if I reply, it goes to whoever most recently sent me a message. The only way I can message the other person is to delete the thread and start over. This has never happened before, just on iOS 12. Previously they'd each get their own thread in messages. My son is on a child account and he's running iOS 12 as well, and has the same issue. My daughter is also on a child account and is on iOS 11, no issues. I've checked my contacts and they aren't sharing any info. I've also checked both their devices and only their email and phone numbers are checked in messages. Any ideas?