invisible spawning mobs

i went on 18w43b to check out the new stuff coming for 1.14, but when i tried to spawn the new mobs they were all invisible.

What I expected to happen was...:
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...