https://www.theverge.com/2018/11/14/18095521/google-pixel-3-xl-disappearing-messages-bug-fix

Are disappearing texts worse than disappearing photos? I don’t know, but both are things that consumers shouldn’t have to worry about when using a smartphone. Unfortunately, both are also things that some Google Pixel 3 owners have encountered. After Google acknowledged and said it would fix a bug that can lead to lost photos, there are also scattered reports from Google Pixel 3 users about their messages disappearing from Android Messages for no good reason. Android Police reported on the issue yesterday. It doesn’t sound like a rampant problem, but it’s still a pretty bad one.

Some of those affected think the November Android security update might be to blame, but others say that their messages vanished sometime last month. Perhaps the problem is tied to Android Messages? 9to5Google notes some users have managed to recover lost messages by rolling the app back to an earlier version, which you can do by uninstalling updates from the app details page.