Hey just so you know, you’re not alone. And a broad search of this issue after iOS18’s release shows a huge ratio of the affected users having issues with their partners’ conversations (my wife and I are experiencing this EXACT issue).
For context - I’m a former Apple Technician of 10 years and a good track record of squashing problems. And I have done everything short of deleting the conversation full of loving memories from my phone and seeing if that works.
There are no duplicate contacts in my contacts app, her info has not been entered by mistake in to another other contacts, we have tried updating to .0.1, we have deleted each others contact info and re-entered them multiple times, each attempt having us enter all the information or just some to see if it would work.
Nothing has helped.
The message preview on the lock screen shows her phone number starting a “group chat” with myself and my wife’s chat, but when I click on it it just goes to the existing conversation with no issues.
During troubleshooting, sometimes it appears as “Maybe: <My Wife’s Name>”, other times it’s just the phone number.
We DID find a dumb, clunky work-around, maybe it’ll work for you? If she goes to her contacts (or vice versa, depending on who has the issue) and she clicks on “Send a Message”, a dialogue box pops up asking if she wants to send a message to your phone number or to your Apple ID; for me, if she clicks my number, everything is fine. But if she tries to message me normally (ie: open messages, type & send or via Siri) then this issue appears again.
It’s as if the iMessage servers are splitting the users apart somehow. It’s ridiculous. This is the literal BASIC level of functionality we should be expecting but iOS18 has become a laughing stock of poor design decisions and unforgivable bugs that should never exist on consumer-level software. A total joke.
Hang in there my brother, and try to submit your complaint at feedback.apple.com