Me and my brother were doing a creative vs survival experiment we were recording, The person in creative, me, was holding bedrock, because you can only get that block in creative without glitches or bugs, while, my brother, was holding dirt blocks, but got killed by the wither. My brother built this wall so he can hide from the wither, and he went to third person mode to see where the wither was. But the wither instantly blasted at the wall he was hiding behind.

How to reproduce: 

1. spawn a wither and get some blocks and go in survival.
2. The wither will attack you so run away 50 blocks and build a wall to protect you.

Expected result: The wither will not see you and will attack other mobs.

Observed result: The wither will attack at the wall that you made as if it can see you behind the wall.

Edit: This does not work in the latest version (1.16.40 hotfix).