PCS are going to drop when you are in TENTH after the short program. The judges can't just pretend the short program doesn't matter. While you may not care for Leonova's skating, or think she is sloppy and isn't as good as Zhang (I agree completely), the fact of the matter is that her TWO programs were overall deemed better than Caroline's, which is why she won by a small margin. Caroline won the long program, and rightfully so, but the judges are saying BOTH programs matter, not just one. Or at least that's what I think they are saying.
Look, we all wish PCS would truly be a reflection of a skater's skating skills, choreography, interpretation, etc, and they usually are. BUT, they are also there to give the judges some leeway, so that the skaters who deserve to win actually win! So, I'm sure Leonova's PCS skyrocketed not because she suddenly has amazing choreography or the best skating skills, but because the judges deemed her performances the best this week! And they felt she deserved to place better than someone who only delivered the 10th best short program.
I personally wish they would get rid of all the separate categories for PCS (since we all know the skater with the highest PCS will have had the highest score across all 5 categories whether they deserve it or not) and just call it the "Overall impression" score. That way, the skaters with the best presentation, skating skills, etc, will still be rewarded, but this mark can also fluctuate based on how well everyone skates that day without it seeming strange!