I have a Samsung Galaxy S8+. Have always had the ability to search an entire conversation from the beginning of the conversation (2 or 3 years) but now when I search it only gives me results from the last 2 months or so. I do know the one contact in particular is an iphone user if that makes a difference.
I personally keep in touch with everyone through messaging myself, so I understand the importance of being able to refer back to previous messages. We will gladly help. Now based on which messaging application you are using and how you have your settings enabled will determine how far back you can view. For example on the application I’m using, all messages over three months are set to auto-delete. When did you notice a difference? What application are you using to text?