Monsters Hunted Advancement not Registering Ender Dragon and Vex

*This occurred in a Realm, let me know if this is better suited for the Realms bug tracker.

I have not been awarded the Monsters Hunted Advancement after killing all 33 required mobs. I only noticed that some mobs hadn't registered towards the end of the 33, so I do not know which ones they were.

What I expected to happen was...:
Being awarded the Monsters Hunted Advancement after killing all 33 required mobs

What actually happened was...:
Two mobs did not register, even though they appear in Statistics.

Steps to Reproduce:
Tried killing mobs in a new single player world, spawning and killing them in creative mode, and I received the advancement.