It has to do with the zombie collider, and the invisible air portion of the block.
To zombie ai it still only accesses it as a cube.
The player is allowed to pass through this but zombie ai is not able to.
For visually partial blocks, it allows for variation from minecraftesque
configurations. But, it wreaks havoc on the the pathing state machine.
I figured it out in alpha 16, when we were coding in the 1 block crawl.
Then visually it was more pronounced as they added more blocks. Now if you
put a ladder segment up to a wall , anything other than a full cube and it will
lock in the air.
Either as you all have stated the, transition should be fixed or the colliders
for blocks need to be moved back to where the visual texture is.
It's the same reason terrain does not mesh up with a slanted curb. Adjusting the
block collider to the texture instead of the perimeter could possibly solve both
situations at once.