Soulsand and wither skulls remain when spawning a wither in the Nether

Put the summary of the bug you're having here

What I expected to happen was...:
Blocks disapearing when I spawned a wither

What actually happened was...:
The wither spawned, but the blocks used in his creation didn't, they could be retrieved too.

Steps to Reproduce:
1. Go to the Nether
2. Spawn a wither using soulsand and wither skulls.