So, I recently thought of this and can't find anyone else who has (so not sure if it holds water or not)
Facts that support my theory: In WoR, Jasnah's shadow pointed the wrong way when she was at the treaty signing. Sja-anat can appear as a shadow that points the wrong way. Sja-anat "enlightens" spren by injecting voidlight that interferes with their Honor and Cultivation bits to make them free thinkers. Jasnah was raised in a strictly Vorin household but rejected the religion.
Theory: Sja-anat tried to enlighten Jasnah as a child after becoming her friend that caused her psychotic episode. This caused Jasnah to become a free thinker that led to her rejecting Vorinism.
I dont believe that she is still "enlightened", because she fought off the voidlight, possibly with the help of Ivory.