Make sure the user shows up in the FileVault users list.  We will need the UUID for the last step.

sudo fdesetup list


Find the Macintosh HD ( or whatever your boot disk name is ) You will need the IDENTIFIER for the next step.

diskutil list

/dev/disk0 (internal, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *41.9 GB    disk0

   1:                        EFI EFI                     209.7 MB   disk0s1

   2:                 Apple_APFS Container disk1         41.7 GB    disk0s2

/dev/disk1 (synthesized):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      APFS Container Scheme -                      +41.7 GB    disk1

                                 Physical Store disk0s2

   1:                APFS Volume Macintosh HD            11.9 GB    disk1s1

   2:                APFS Volume Preboot                 44.2 MB    disk1s2

   3:                APFS Volume Recovery                509.6 MB   disk1s3

Run this command to sync the FileVault and login passwords.  You MUST know the old password.  It will NOT work without the users old password.  (disk1s1 is the identifier and 68C6BCDD-9F15-4449-B38D-63E2571ECD9F is the UUID)

sudo diskutil apfs changePassphrase disk1s1 -user 68C6BCDD-9F15-4449-B38D-63E2571ECD9F

Old passphrase for user 68C6BCDD-9F15-4449-B38D-63E2571ECD9F:

New passphrase:

Repeat new passphrase:

Changing passphrase for cryptographic user 68C6BCDD-9F15-4449-B38D-63E2571ECD9F on APFS Volume disk1s1

Passphrase changed successfully

Restart your Mac.  The Filevault and Login passwords will now be in sync.