I can tell you when it triggered for me the one and only time and whether it has anything to do with the location or just pure randomness.
After visting the plague ship in Act 3 and choosing to avoid it by going back to camp, it was a habit that if I talk with Minthara I'd give her a kiss and in that particular moment the poison kiss was triggered.
It was probably all just random but also was during patch 6.
