How do I recover hidden messages from a 1v1 live chat?

1. Find the hidden user's profile through an ID search or your Gifting list.
2. Select "Send Message" in the user's profile and send any message.
3. Return to the 'Messages' tab, and the chat will reappear.

If your entire history is empty, it might be a cache issue; please see Why did my Chamet chat history with a friend suddenly disappear? | Chamet Help.