Wither boss bug when peaceful

When you try to create a wither in peaceful mode, some blocks remains
To reproduce:
1-Go to peaceful
2-Create a wither(with soulsand and skulls)

After it, i removed all blocks, and placed again(in hard mode). The wither spawned. I changed back to peaceful while the wither was "charging" and it desapeared. Then I tried to create another one at the same spot and it fails.