I kind of figured it might happen, but I think it's really kind of funny. This only occurs on sufficiently high enough missions or at certain breakpoints though(I think).
Rather than reducing missions by a % amount, FP is reduced by a set number when the mission has the special condition "Reduced FP/EP Consumption"
For example lets imagine a 14 FP. With this condition, it might become something like 14-4(the reduction)[I don't know the actual exact number], for a 10 FP cost, 5 FP to enter and 5 FP for killing the boss.
The current event slashes FP costs in half then rounding up. So a 14 FP mission would be 8 FP. But since the "Reduced FP" condition is on, it becomes nil.
*well, the exact math probably isn't like this
tldr: Reduced FP district missions only cost 1 or 0 FP, which has an insane exp ratio efficiency
