Create Rotten Zombie 0 exp!??!

IGN: ToxicDemon_  
Level: 42
Server: Every server

Problem: Rotten zombies are sometimes dropping 0 exp!?!? 
Evidence: https://www.youtube.com/watch?v=liWnKdF7yxs&feature=youtu.be

Please msg me on the forums if this is a bug or not.