Zombies can't damage a villager for many times when both are enclosed in the same 1*1 cell made of iron bars

When I use the spawn eggs to spawn a villager in the cell and then spawn a zombie in the exact same position, the zombie only hurts the villager once or twice, and then it seems to ignore the existence of the villager. However, the zombies outside of the cell and still see the villager, converge on the cell, and even hurt the villager through the iron bars. (The blocks on top of the bars ARE necessary to replicate the bug because they prevent the villager from changing its y-coordinate when taking damage.)

Note:
The bug doesn't seem to work when you spawn the zombie before the villager.

The screenshots are from a MODDED minecraft and I am aware that this bug tracker only deals with problems with the original, unmodded minecraft. However, I DID test it with vanilla minecraft, in which the .minecraft folder and 1.6.2.jar and 1.6.2.json are BRAND NEW, UNCHANGED. (I also tested it with 1.5.2, in which the files are also brand new.)