FileVault-enabled MacBook asking for old password after resetting via 'resetpassword' in recovery mode

codedil wrote:

FileVault was enabled and FindMyMac was not enabled.  

It appears that you have forgotten your FileVault password. This is much more serious than forgetting a user login password, for which there are various solutions.

Apple's documentation warns about this and offers two methods to protect against a user forgetting their FileVault password: Protect data on your Mac with FileVault - Apple Support

See especially the sections I put in BOLD below:

Protect data on your Mac with FileVault

If you have a Mac with Apple silicon or an Apple T2 Security Chip, your data is encrypted automatically. Turning on FileVault provides an extra layer of security by keeping someone from decrypting or getting access to your data without entering your login password. If you use a Mac that doesn’t have Apple silicon or the T2 chip, you need to turn on FileVault to encrypt your data.

To set up FileVault, you must be an administrator. When you turn on FileVault, you choose how you want to unlock your startup disk if you ever forget your password:

• • iCloud account and password: This choice is convenient if you use iCloud or plan to set it up—you don’t need to keep track of a separate recovery key.
  • 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.

Apple is very serious about the above, when they say "lost forever" they do mean that, hence they provide two ways to prevent this from happening. I am surmising you did not set up either of those two options to mitigate a forgotten password. The third option of course is maintaining complete and redundant backups of all files (I think you indicated you do not have a backup either).

I will not lecture you here, but as an engineer I do know when to recognize that something is physically impossible. Decrypting a FileVault protected solid state drive is, I believe physically impossible, not just for the user or for Apple, but also for any number of government agencies that specialize in such things.

This is why I do not use FileVault for my personal Macs. I also have at least 4 backups stored in various different secure locations.

My employer requires FileVault on all its Macs. But these are also "managed" Macs and the employer maintains its own "key" for each and every one of its Macs because employees do these things like forget their FileVault passwords.

I think what everyone is trying to tell you here (including Apple itself) is that without one of those two "forgot FileVault password" options, there is actually no known way for anyone to mitigate this.

Usually when a disaster like this happens, the user does eventually recover some of their files through:

• cloud storage (do you have Photos in iCloud, Messages in iCloud ...)
• emails and text messages sent to others with attachments, e.g. photos, videos ... these can be recovered from those others
• other copies made to external media or other cloud services (Dropbox ...)
• files, photos, videos on your iPhone or other cell phone, or other devices (tablets, even Apple Watch ...)

I would encourage you to explore the above to see what can be recovered.