Darth Malak possible bug?

I play arena againts Darth Malak using jedi Revan. Darth Malak unique writes :if he suffers damage over time or critically hit he inflicts pain. in my experience this is random, when I hit him with grand Master Yoda having 100%crit buff he doesn't get pain 90% of time. anyone can explain? the ability clearly says can't be evaded or resisted.