Please feel free to move/delete this if it has been already discussed. I, for my part, didn’t find anything regarding this topic.
I have 120% ignite chance from “Phoenix Strike”, 172% from “Eye of Reen” and 8% from “Rahyeh’s Devotion”, sums up to exactly 300%. Now with “Apocalypse Whirl” it gets converted to Time Rot. After Warpaths reduction to ailment application we have 180% (300 * 0.6). Now we add the 30% chance from “Apocalypse Whirl” (which should not be affected by the 0.6) and should land at 210%. This, as I understand it, should give me a 100% chance to inflict two stacks of TR and an additional 10% chance of inflicting a third one. But when testing this on the dummy I get two stacks for around 95% of the time and one stack for 5% of the time. With 210% it should be impossible to only get one stack, right? And if you do the math with Apocalypse Whirls 30% chance also getting the 0.6 reduction, you land at 198% (330 * 0.6) which would come pretty close to my observations.
Furher, if I add an item with additional 20% ignite chance, I finally get the resulsts I expected with my first setup: 2 stacks for 90% of the time and three for th rest. Those 20% push me over the 200% mark if Apocalypse Whirl is also impacted by WPs reduction (350 * 0.6 = 210).
So I don’t know if I have a poor understanding of the ailment-mechanics (I did some research though and I couldn’t find anything that disproves it) or if it’s actually a bug, it seems like one to me.