I was about to place this topic in the Unite and Fight/February 2019, but I am certain the case applies for any high level foe.
I don't think that the debuff success rate stays the same probability percentage. Rather, the level 75+ foes have a higher debuff resistance. I usually have as much characters, weapons, and summons to debuff attack and defense. I also tried to have innate debuff success in the extended master perks. Maybe I need some debuff success buff?
Is it a good idea to keep using