All Curses now have a line which says (example Level 12 Despair)
Curse does not apply to enemies above Level 61
If it felt like your curse randomly stopped working it's probably this line affecting you. Maybe you forgot to level your curse, maybe your Stat requirement held you back.
I've thought about and I really don't like this change. Fine, you don't want a character to enjoy the benefits of a powerful curse without investing in it, and curses are insanely strong!
Now an endgame character will only use the Max level curses and they either have to go "all in", dealing with the high mana cost, and raising that required stat to >150. It also makes more creative archetypes harder to