With the release of iOS 12, a few users online have posted about issues where their iMessages get combined or merged between different contacts with different phone numbers. This is not like a group chat, as in my experiences with this issue, sending a message on the rogue text thread would send to either person at random. Incoming messages from either person would go into the same rogue thread with the contact name and phone number changing to the person who last sent me a message. Moreover, I tried to control who messages were sending to by composing a new message and manually typing in the phone number, but that would only bring up the rogue combined thread and send as part of that, so I had no real way to text one of the people at any given moment. This was especially frustrating as the people who I couldn't text were my parents!
To make this even a little more confusing, if I deleted the rogue thread and sent new messages to make new ones, texts would work as normal to each number until I restarted my phone. Then the threads would combine again. This was good in theory; however, it was not a permanent solution that could be relied upon, as I would just have to delete my texts again if my phone restarted and perpetually not have any texting history. In the meantime, though, I did have a group chat in iMessage with myself and them that was still working, but again, sending in that forever is only a workaround.
Luckily, with the help of Aleisha, a senior advisor at Apple Support, I was able to get to the bottom of this issue and fix it without too much data loss, and you can too.
This iMessage issue is not widely reported (perhaps for factors that I will list below), but for the sake of posterity, here are all of the bug reports about this issue that I have been able to find online. Do not be surprised if you see a link to this webpage on each of these links, as I intend to spread this fix as soon as possible.
This iMessage issue started happening since iOS 12 attempts to combine threads if emails or phone numbers are on the same Apple ID. (The linked screenshot reads "Your Conversations Streamlined: Messages will be kept in a single conversation when you send or receive from linked iMessage addresses").
This is speculation, but I believe that as more people have adopted iOS devices since Family Sharing and other features like it were released, people have naturally begun to set up separate Apple IDs for each person who uses Apple products. I have found that, in my personal experiences, most families have separate Apple IDs per person. Just because of this issue alone, people will need to quickly get their own Apple IDs if they are still using a shared one, making cases like this probably even less widespread in the future.
Here are the steps that you need to take to fix this issue. They will be tedious, but you will not lose too much data if you follow these steps exactly. Do note that some of the steps are carried out on the devices that you (the person with the issue) do not own. Those other people (who you are trying to text but cannot) will now be referred to as "recipients" for the rest of these steps, and steps will clearly be marked when recipients or their devices must be involved.
Once the sync is complete, send new messages to each affected recipient to make a new thread. You should also ask for a response if you can. Once you've sent messages and hopefully gotten a few responses, restart your phone. You should now hopefully still see individual threads for each person indicating that this issue is completely fixed.
If you have other information relating to this issue for the sake of posterity, or if you are experiencing issues with the steps that I've outlined on this page, please let me know. My contact information is available on the homepage.
Page written on 25 September 2018 and last updated on 8 October 2018.