The OxygenOS Open Beta 3 was released for the OnePlus 8 and 8 Pro just before the end of October, but there has been a certain mix-up. Some users received the stable update, version 18.104.22.168, instead of the beta, and installing that has led to a full wipe of data.
If you’re one of the users who have received the wrong update, but haven’t restarted the phone yet, you should download an APK, a specially-designed app that will help preserve your data. All you need to do is install it and hit the “Keep data” button.
However, even with the APK, it’s best to use the OnePlus Switch app to back up your data and move that backup off the device for safe keeping. Also, note that some users are reporting that their phone automatically installed the update, and rebooted, wiping out their entire data in the process. So, if you’re on the Open Beta channel, it’s best to create an update.
The official OnePlus Forums post has more details on the issue and its resolution. There are other issues with the OB3 release as well, including users reporting troubles with 5G when using two SIMs.