text messages for just one contact keeps disappearing

All of a sudden, one contact's text messages simply disappeark across all my devices. It's not a settings issue as I have imessage set to keep forever. I am not experiencing this issue with any other contacts. The preview will come through, I click on the message and it goes to a BLANK screen. I've tried all the suggestions, signing out of imessage, re-syncing, turning off/on wifi, complete/full reboot of my iphone, deleting the contact and starting a new string - to no avail. If this contact was anyone other than my ex-husband and father of my kids, it wouldn't be so imperative to get this fixed, however, it's simply unacceptable for this to be happening. And it's only with direct messages between us. If he's on a group message with our kids, the messages are fine. What is the issue and how do I resolve this for once and all?