You can follow the feature on github at https://github.com/jefftharris/passwdsafe/issues/14. I'm fairly close to having basic support. it's more a matter of finding time on the weekends.
I would suggest saving from the email to some location in your internal storage. The email will be subject to the email app providing the file attachment which could cause the inconstancy.
The app only manages the backup files itself. They are in a private app directory that you shouldn't be able to access. The app doesn't do any special management for the user file. New files normally are created in the top directory of the internal storage, though the user can choose some other directory. I'd suggest opening and checking any file before deleting to make sure it's not one you need.
Unfortunately, there's no built-in backup of the full file. If you modified it before, there may be a backup you can restore. Check the top-left menu for the backups. If there's a file, you can open it and restore.
passwdsafe: LauncherRecordShortcuts: Cleanup file data view properly
passwdsafe: Use ViewPager2 with PasswdSafeRecordFragment
passwdsafe: PasswdSafePolicyListFragment: Convert to MenuProvider
passwdsafe: Use fragment results with PasswdPolicyEditDialog