Whenever I spawn a witch or wither, they dissapear almost instantly.

What I expected to happen was that i could fight and defeat a witch and a wither:

What actually happened was that the witch and wither dissapeared 1 second after being spawned.:

Steps to Reproduce:
1. get a witch spawn egg or sould sand and wither head
2. spawn either the witch or the wither
3. whichever is spawned will dissapear. the witch dissapears if spawned, the wither dissapears if spawned.