My Mid-2012 Macbook Pro 13" (Core i7) with Mountain Lion (10.8.5) always asks for the FileVault recovery key (consisting of 24 digits and letters) when awakening from hibernation. I just get the options "Disk Password" and "Guest User", but cannot use my user account to unlock the drive. After entering the FileVault key, my Macbook shows me the normal login screen, where I have to enter my user password.
My main user is of course allowed to unlock FileVault in the FileVault preferences pane. Hibernatemode is set to "3". I've installed an SSD drive, but I think (?) this behaviour has been there even with the original hard disk.
One detail is additionally surprising to me: Until recently I used this Macbook as my main computer and thus it's nearly always been connected to an external display when doing some longer sleeps e.g. during the night. As I now use an iMac, the Macbook no longer is connected to a monitor and every day I awake it, I'm asked for the recovery key. This happens even if the Macbook is connected to the power supply all the time.
This is how the login screen looks: