I have submitted a bug to Apple about this. It's infuriating me beyond measure. And I'm a huge Apple fanboy. Apple MUST fix this. I upgraded to iOS 12 on our 4 iPhones (my X, wife's 6s Plus, boys' 2 5s's), all our iMessage/Text messages are now merged and using the same thread. I didn't ever see any message asking if we wanted to merge anything--it's possible one of my boys did and tapped YES. Either way, what a ridiculous "feature". I try to text my wife by specifically typing in her name or number into a new message then when I send the message, it goes to my son instead. But if I scroll up in the text history, it's the year-old text history I've had with my wife all along, but now I'm having the conversation with my son! It's just wrong! Then my wife IS able to text me and the contact picture of my son now changes back to my wife's and I'm back in a conversation with her. This is so messed up. I called Apple support last night and spoke with tech support for 30-40 mins about it. The tech recommended many things, but overall, to create new Apple IDs for my sons and my wife. I did not want to do this, but I did it anyway to try and fix, separated out everyone with their own Apple ID, got one of my boy's his very first email address, reset settings on iPhones, rebooted, turned iMessage off and on to re-register it, deleted all of the other 3 people's contacts on each iPhone, rebooted some more, and it still did not work. I've been sharing my Apple ID with my wife for years now, and for the last 2 years with my 2 boys' 5s's. Haven't had any problems with it. I know ALL about the "Send and Receive" setting in Settings > Messages, but this is beyond that--that's not the issue here. This is a mess and Apple dropped the ball big time on this one. They need to fix it ASAP--especially since I have now separated out each iPhone into their own Apple IDs and the problem persists. Only thing left to try after I get off work tonight is going into each iPhone and deleting all the texts and group texts for each of the other 3 contacts. Hopefully that can clear it up!
If anyone has any ideas or suggestions for me, please let me know. Sorry this was so long, but I'm just furious about this change they made, intentional or not. Thanks!