Devs: Make "Raise the Flag" mission available in higher difficulties.

Just did my first trival mission to help a friend and saw this objective. After investigating it seems to only be a mission in difficulties 1-3. Why?

This is probably one of the most patriotic, awe inspiring missions in the game, and I'd say 80% of players will never get to see it or hear it's OST which is one of the best in the game. It's a shame and should -be a mission available in all difficulties.