I have the same problem but mine is 32 of 33 Monsters Hunted. (Attachment) I have checked Wiki and according to me I have killed all 33 hostile mobs in the list.
I also check Statistics/Mobs and the list confirm that I have killed one of everything in the list.
Hercules Oberholzer
Ender Dragon is missing.
I did however kill the Ender Dragon and the Statistics confirms this (Attached)