Unread messages eventually disappear from Notification Center

I have been dealing with this issues for years, since iOS 7.1 update. I now have an iPhone X on iOS 11.1.2 and my messages still disappear from my Notification Center. If they’re left unread for long periods of time (or sometimes even just a few minutes) they disappear completely. I’ve done so much trouble shooting for this over the years: I’ve gotten new phones, updated my software, reset and restored the phone, set up the phone as a new phone from no backups, changed notification settings, countless apple store visits and phone calls to apple... but, nothing avails. Has anyone else ever dealt with this, or have a possible solution? Updating software doesn’t fix it.