I thinks it simply that when she when in to the story those whole where simply character became actually ppl with complex personalities and emotions
Well the fl get into the palace as a concubine, and all of the concubine literally racing with sweat and blood to get the emperor attention, and because the ogfl also tried to get the emperor's love they see her as another competitor. Meanwhile the current fl just straight up reject the emperor attention hence all of the concubine lower their guards on the fl and sees her as no more than a child being forced into the palace.
So....remind me again if I'm missing something but why is everyone who're supposed to be horrible people so nice to FL all of a sudden???? Like did the fl knew wrong information about the og fl or something??