macOS File Vault 2 full disk encryption does not seem to work
It looks like this is a misunderstanding. In the later versions of macOS it is difficult to distinguish the pre-boot unlock screen (where one unlocks the partition and is automatically logged in after the boot process) from the login screen (where the system partition is already unlocked and the user has to log into the system).

Furthermore, FileVault2 can grant multiple passwords the right to unlock the partition and grants this permission to individual accounts. You can manage what accounts can unlock the system disk in System Preferences>Security &amp; Privacy>Enable Users.... That means your account password also becomes the password to unlock the partition. In any case you can use the recovery password, which is provided when encrypting the disk, to unlock the partition.

This answer provides a method to use a unlock key that differs from your account password.