Time Machine backup to encrypted disk
FileVault is for internal startup disks; it sounds like you've just got an encrypted external disk. It's possible when you set up the encryption on the external disk, you saved the password in your Keychain, so in the future macOS reads your keychain for the password to the disk.

If you can't remember the password and don't have it elsewhere, then you cannot access the data.

Put your previous SSD back in your Mac, and either connect the external disk and decrypt it, or inspect the keychain to obtain the password for the disk.