Hiya, bro, did your iPhone messages would disappear without any reason?
Though I don’t know how to exactly solve this issue, I can give you some tip to try to fix it.
you can try to force restart your iPhone, or enable and disable your iMessages on iPhone, and check your message history settings also are okay.
Next is telling you how to fix disappearing iPhone messages issue.
Usually we would choose to recover disappeared message from iTunes or iCloud, but that would make your current data lost, so if you want to keep your current data, maybe you can turn to the help of third party tool.