iPhone text message group now has two separate chains?

iPhone 15 Pro. Just updated to iOS 18.1.1

My text messages have gone haywire. Please stay with this, it's a little convoluted but I need a fix for it.

Here's the chronology:

I have had a group text chain on this phone for at least a year, and until today it has worked fine. A couple days ago I did an update. Today I received a text from the group, but it came in as a brand new message, not connected to any of the group texts that had gone before. Everyone from the original text chain was on it, but it was not named the same as the original chain. We had renamed the message chain but the new text just showed the phone numbers of the people on the chain, even though they're all in my contacts.

A few texts back and forth then stayed on the new chain while the original one was unaffected. I confirmed that the sender of the first message had sent it from the original chain, and I asked whether anyone else was experiencing the same thing. Another group member also said they were seeing two different chains with the new message. We tried renaming the new chain to match the old one, and for a brief moment it appeared to merge them. The new single chain now had the previous text history, minus the new texts where I asked about what others were seeing.

I thought maybe restarting the phone would help, so I did. When I opened it up again, there was only one chain. It had the original name attached to it, but it only had the most recent series of texts. The entire previous history was gone. I sent a message stating this to the group.

Next, the person who also said they were seeing two chains replied to the group, saying they were replying from the original chain, even though my messages were coming to them on the new one. Their reply showed up as a brand new text for me, also without the original name, but when I opened it, not only was the new text there, but so was the entire history, minus the recent texts discussing the new text chain.

Obviously, it's annoying to have to navigate two text chains never knowing where the next text will pop up, not to mention the potential for losing important stuff again. This is a work text chain so we share information that we refer back to regularly.

Can anyone advise why this might be happening or suggest fixes to try?