Since 17w47a, blue wither heads can fly away through blocks on the first tick(s?). When looking at the first screenshots provided, we can think that the two neighbouring wither skulls are close enough to the edge of the 3x3x1 bedrock surface to shoot blue wither heads away from the surface. But in fact, those blue wither heads are just going through blocks. When looking at the next screenshots, you can notice that the wither can shoot projectiles underneath the 3x3x1 surface. This wasn't the case before 17w47a.

Steps to reproduce:

• Build a 3x3x1 bedrock/barrier area
• Put a fence 2 blocks underneath the middle block of the 3x3x1 surface
• Put an obsidian block 1 block higher from the middle block of the 3x3x1 surface (or a normal block but don't forget the water on the wither main head to prevent it from blowing it up)
• Spawn a wither on the fence (horizontally)
• Place blocks around the 3x3x1 surface and hit the wither with projectiles (snowballs)

I've provided screenshots and a world download to help people reproduce this bug .