Annoying mechanic, or an oversight. Buffs aren't removed when monsters die, but I'm not sure why the list isn't at the very least being trimmed to be displayed the way player's buffs are ("something something buff x 30", instead of a column of buff names).
I kind of like when monsters have that. You look at this monster you just killed, and it has twelve curses and burning and acid and thaumites on it and you think to yourself, "hell yeah, i killed that monster good"