The wither can see through blocks

I was fighting the Wither recently.  I had setup a little bunker in the ground made out of obsidian. I spawn the wither and start fighting it. My. health was low so I decide to run into my bunker. As I'm doing the wither follows me and starts shooting at me. No problems there. The bunker is set up so there is a long hallway for me to run before getting to the bunker, so the wither can't just blow it up. But as I'm running I still hear explosions coming my way. The wither is actively following me while I'm underground. Once I make it to the bunker, it blows up the base and kills me. I go back and collect my stuff, and am once again low on health. So I dig my self into the ground and dig forward so the wither won't know where I am. Again, it follows me and kills me.

Steps to Reproduce:
1. Dig a long tunnel into the ground
2. Spawn the wither
3. Go into the tunnel and start running

Observed Results:

The wither follows as you run trough the tunnel (even though you're covered with blocks) and blows up the top of it, until it reaches you and kills you.

Expected Result:

The wither follows you to the entrance of the tunnel, possibly blowing up the entrance, but then looses track of you as you disappear into the tunnel.