Bifrost (chapter 11) goal does not update on "extreme" difficulty
After the very first attack of firegiants directly few seconds after the game starts, on "normal" and "hard" difficutly you will get a sequence showing giants attacking the bifrost and you will get the mission to prevent them from destroying it.
But on "extreme" you do not get this mission. I lost after a while because the bifrost was destryoed, but I did not even know that I had to protect it.
This is easy reproducable.