Why are old received text messages disappearing from iPhone?

In researching old text message threads for a client I have found that there are missing text messages I know existed in the past but are now no longer there. These are messages that were received, not sent. I have previous screenshots proving their existence, but can't share them due to confidentiality. According to apple the messages can be unsent within minutes of sending them, but the timeline says it's been months. How could this have happened?