His miss can be attributed a bit to the clever fake by Szczęsny.
Watch this in slow mo from 2:19
https://www.youtube.com/watch?v=7CY9S78Ekew
https://ibb.co/jW6rCQX
Szczęsny moves his weight towards his right. kessie's technique is to adjust his shot based on gk's move. but it turns out to be a fake from the gk and hence the penalty save.
full gif:
https://giphy.com/gifs/kessie-penalty-vs-juve-kgO3cS85l4bMFuvVBZ/embed
I was a fan of Szczesny since his starting at Arsenal. He always does that ?litlle dance shake? before the player kicks it. This time helped him buuut..i will be a hater of juve stealing over the years and say that his foot and dive is kinda to far from the line so they should have repeated it