Make sure the user shows up in the FileVault users list. We will need the UUID for the last step.
sudo fdesetup list
bob,68C6BCDD-9F15-4449-B38D-63E2571ECD9F
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.