I've had this issue about 2-3 days after updating my phone to iOS12. No issues at all with any precious iOS. My husband, 2 kids and myself are all on the same account. We all have iPhones and have messaged with no issues until now. After the update I noticed when I was trying to send a text to my son it was sending it to my husband. Honestly I thought it was me just hitting on the wrong name but after I really watched what was going on it would say my son's name on the top (who the text is going to) but when I hit send it literally changes that name to my husband. So this is what I have figured out. When I go to messages if I don't see all 3 names listed as people I have texted previously and only one (it changes between the 3 of them) then it doesn't matter who I want to send it, it'll only send to that one person. I have to delete that thread and start a whole new one for it to go to the correct person. My husband and I have updated to the iOS12 but my kids have not. But it seems I'm the only one with this issue at the moment. I really hope this wasn't too confusing lol.