When you start a Story mission you can choose between Story Mode or Challenge Mode,
On Challenge Mode you can choose between several Options that increase the PR for the Mission.
But, on ALL other missions(including Tarot) the "modifier" for the difficulty from you last played Story mission applies without any option to manually change it.
The only way to change it is to play another Story mission on a other difficulty, but then again those difficulty applies to all other missions.
This makes Tarot mission especially painfull, because they are either too high(Challenge mode Normal onwards) or to low(Story Mode)
I hope you can fix this asap