Hello there, the feat Greater Dirty Trick states:
"...Whenever you successfully perform a dirty trick, the penalty lasts for 1d4 rounds, plus 1 round for every 5 by which your attack exceed your target's CMD."
Seeing as I'm playing combat maneuver rogue, I've used dirty trick A LOT of times, yet if I exceed target's CMD by say 2, they always get the debuff for one round only. It is statistically so improbable for it to only ever get 1 out of d4 so I'm pretty sure that this indeed is a bug. This means that they are nearly useless agaisnt high CMD targets, as they get the penalty for one round only and I cannot sneak attack them the next round. I hope the developers monitor this forum, as fixing either the description or the bug would be great :)