Just found out that Yuyuko's Ghastly Dream is bugged, instead of decreasing every enemy's ATB by 25%, it decreases only the ATB of the target enemy by 25% per the number of enemies (so it reduces their ATB down to 75%, then to 75% of that value and so on). This also seems to have a very weird interaction with bosses, since they halve ATB reducing effects, and in this case, it seems the amount gets reduced by half for every instance they get hit by the effect (so first by 12.5%, then by 6.25%, etc).
EDIT: After some more testing, it seems that my numbers on the last part were incorrect due to the fact that the reduction on bosses is different for each spell, while bosses reduce the effect of Ghost Butterfly to lowering 15% ATB instead of 33%, they reduce the effect of Ghastly Dream to 8% from the original 25%. So they simply get that 8% reduction multiple times if there's several enemies.