Given that a zombie can't jump to the same elevation as a villager, it won't attack it.

There's not much to be said; to reproduce make sure the zombie is one block below the villager and can't jump to the same elevation. If done correctly, the villager will never be harmed regardless of proximity to the zombie.

To demonstrate the severity of this issue, I pushed a glass block into the lower half of a zombie and trapped a villager directly on top of it.  Again it was not harmed.

It's worth mentioning this bug does not persist if the zombie is in a minecart.