Steve Jones implores you to test those database backups by restoring them somewhere:
What do you do? Hopefully you recognize the issue and can fix the issue. Maybe more importantly, you have a backup of the missing certificate.
Most people don’t deal with encryption, but you never know when your backup job might start failing, perhaps writing to a damaged file that appears to work (if you write as a device) but really isn’t capturing the backup file. Perhaps you don’t know that your backups are being written to a location and deleted a day later, but the process that is supposed to copy them to tape or a remote file share is broken.
Any number of things can happen. The point is that you want to be sure that you are actually getting useable backup files.
That means testing restores.
Read the whole thing.