Since no one helped me here, I figured out how to unlock the achievement by myself. Maybe it will help someone.
The problem is in interaction between Epic layer and the game, I guess. You have to "initiate" the layer by pressing Shift+F3. After that achievements are unlocked.
So, I loaded the mission save, where I already completed all optional objectives, then pressed polaroid. The layer checked that the conditions of the achievement were met and unlocked it.