Wither doesn't spawn on X axis.

I'm on my survival world, and I build a wither. I expect it to come to life like it usually does.

However, it didn't. So I tried putting it on the Z axis and it works.

To reproduce:
Build a wither configuration along the X axis of the map:
0=air, 1=not
Z
0 0 0
X 1 1 1
0 0 0