Ventura - File Vault Encryption Key

Encrypt Mac data with FileVault

• Recovery key: The key is a string of letters and numbers that’s created for you—keep a copy of the key somewhere other than your encrypted startup disk. If you write the key down, be sure to exactly copy the letters and numbers shown. Then keep the key somewhere safe that you’ll remember—but not in the same physical location as your Mac, where it can be discovered. If your Mac is at a business or school, your institution can also set a recovery key to unlock it.
• WARNING: Don’t forget your recovery key. If you turn on FileVault and then forget your login password and can’t reset it, and you also forget your recovery key, you won’t be able to log in, and your files and settings will be lost forever.

Then have a few suggestions ;

1 - Get Support Choose a product and we’ll find you the best solution.Start now and open an Apple Support Ticket as they are Apple Employees to deal will these types of issues . 

2 - Product Feedback - Apple and make it known to Apple regarding this ongoing issue 

Q - " Do I need the encryption key to restore his desk from back up if I still know the login password I

A - Enabling FileVault can only be done from the " Owner Account " of this computer.

A - This implies the FileVault was attempted from a Non Owner Account ?

The " Owner Account of this computer "  has that privilege to add, remove, upgrade or downgrade accounts. This would include adding more Admin Accounts and / or downgrading an ADDED Admin account 

Set up users, guests, and groups on Mac

Normally the " Owner Account  " is the very first Account used when the Computer was Very First Setup as New and out of the box.

If the user is attempting the FileVault enabling from an ADDED Admin Account - Log Out of that account and Log Back into to First Account used on this computer when it was First Setup as New.

Adding additional Admin Account can be done there after.