No the emperor and rose was supposed to drink the poisoned tea together as planned but rose was given a jar of antidote. Anyway, at this point rose already knew who's responsible for killing her family so she decided to drink the poisoned alone instead without taking the antidote as you can see in the previous chapters that the jar full of antidote was left untouched. Hence, why she lost her memories because one of the side effects of the poison is memory loss. The tea that the emperor was making her drink was by the time she fully lost her memories was actually n antidote so that she'll recover her memories. Thus, leading us to the point of flashbacks
THEN is sullivan poisoning her coz she is drinking that pink tea?