Portion of Messages History Gone

On the Messages app, for one specific person, my messages are disappearing in an odd way?

I can see the past 50 messages or so by scrolling up, but then it jumps the message history to August 19th? This is constant, when I continue to text, messages past the “50 message limit” disappear. And the messages above that always start on August 19th.

I could search up messages, and messages between Today and Aug 19th do show up. But when I click on them, it just jumps to August 19th again.

Again, this is only happening with one specific person. What’s even going on??

(The only “message history” setting I have is to delete messages a year old. But that shouldn’t be the root of this problem).

I’m on an iPhone 14, on iOS 18.0.1.