I have 149/150 jokers unlocked/discovered. I am only missing the Stone Joker in my collection.
I made the mistake of 'discovering' the Stone Joker in the Medusa challenge first which I understand does not count towards the collection. I now try to unlock/discover the Stone Joker in a regular run but he no longer comes up as undiscovered and when I use him it does not seem to register it (I assume because it was already 'discovered'). Therefore I am stuck with this and do not know what else to do.