Replies in text message thread are suddenly missing

iPhone 7 (Product) Red, 128GB, 107.33 available. 11.4MB of 50GB used on my icloud. Carrier is Verizon.

I have a message thread with someone that goes back to May of 2017. I was looking for a specific message and when scrolling up I realized that while every single one of my sent messages are present, the other person's replies are missing. These missing replies start on December 22, 2017, and continue backward to the beginning of the thread. Our conversations from Christmas of last year onward are all complete. I wondered if it had something to do with the other person getting a new phone but that was in January of 2018 so I'm sure if/how that could've affected anything. I have gone through equally long message threads in my phone and this issue is not present in any of those conversations.

I took my phone to Verizon, they were lost as to what the problem could be but suggested I upgrade my icloud account. This has made no discernable change