Wither In Peaceful spawns very glitchy

I was trying to spawn a wither in the end, on peaceful, and it left one head with soulsand below it.

What I expected to happen was...:
Nothing would happen when I tried to spawn wither in peaceful.

What actually happened was...:
Instead, it didn't spawn, but one of the heads remained, with a block of soul sand below it. Sometimes the head turned into a skeleton head.

Steps to Reproduce:
1. Turn game to peaceful.
2. Try to spawn a wither.
3. Notice the remaining blocks.