There are two possible causes:

Your phone has been restored to factory settings.

Files in Safe can't be backed up. Restoring your phone to its factory settings will erase these files. The missing files can't be retrieved. It is recommended that you move your files out of the safe before backing up data in preparation for a factory reset.

Files in Safe have been deleted by a third-party cleanup app.

Files in Safe will be detected as large files by third-party cleanup apps. When files in the safe have been encrypted, they are assigned random names. Because of this, they are likely to be deemed as junk files and deleted accordingly.

Note:

To prevent files in Huawei pre-installed apps from being deleted, it is not recommended to use third-party cleanup apps. Using Optimiser is recommended.