A relatively small subset of Google Pixel 3 owners have reported that their devices are randomly deleting some or all of their SMS and MMS messages, and Google has now acknowledged the issue and stated that a fix will be rolling out to affected users in the near future. The issue has seen affected devices delete entire message histories or single messages at complete random, with no clue as to what's causing it. Unfortunately for affected users, all that can really be done at this point is to restore any manual backups you may have made, and just sit tight and wait for Google to issue its promised fix.
Background: This mysterious problem seems to only be affecting Pixel 3 and Pixel 3 XL units for the time being. Scattered user reports all have the common thread of text messages going missing, but that's mostly where the similarities end. At least one user has reported that the sync function that syncs together their Google account and their phone is to blame, as it continually wipes manual backups that are loaded. Other users in places like Reddit have reported only a few stray messages going missing. Some are reporting the problem affecting them no matter what app they use for SMS and MMS messaging, and others have said that potential fixes like clearing app data for Android Messages if they're using it has worked for them. The recent and fairly limited nature of the problem makes it difficult to research in any real depth.
Impact: The subset of affected units among the grand number of early Pixel 3 and Pixel 3 XL adopters seems fairly small, but this is a rather grievous issue to have, all the same. SMS and MMS messaging is a fairly primary communication method among the vast majority of smartphone owners these days. It's worth noting that scattered reports thus far do not seem to indicate that this problem exists with RCS messages sent and received through the Android Messages app. Since the problem exists across apps among the set of users putting out reports, it's likely safe to say that something that SMS and MMS messaging have in common is involved in the bug, and other forms of messaging, such as data-based Hangouts and Facebook Messenger communications, will not be affected. One part of the issue that's especially troubling is the fact that many of the users that this is happening to may not have a manual backup to fall back on, and are in some cases losing years of collected SMS and MMS messages that may have been synced from older phones that no longer work or are no longer in the users' possession. In these cases, while Google can issue a fix to prevent the phones from wiping future text messages, it's quite likely that these users will never see those cherished old messages again. Since the automatic backups happen on Google's servers, of course, it's also possible that Google's fix will include helping users to retrieve their old messages.