I'm still seeing this on 1.8.1. It can be reproduced as shown in the attached images. A mess of zombies are spawned, and swept by water over the edge of a 2-block drop and onto a row of magma blocks. The adults die, but nearly all the babies are able to squeeze off the sides of the magma block enough to survive. The effect doesn't seem to happen if zombies are spawned directly on the magma block.
I have found that if a carpet is placed on top of the magma block, the effect goes away. It seems the babies can't squeeze off the sides then, and they die along with the adults. So there is a work-around.
I'm still seeing this on 1.8.1. It can be reproduced as shown in the attached images. A mess of zombies are spawned, and swept by water over the edge of a 2-block drop and onto a row of magma blocks. The adults die, but nearly all the babies are able to squeeze off the sides of the magma block enough to survive. The effect doesn't seem to happen if zombies are spawned directly on the magma block.
I have found that if a carpet is placed on top of the magma block, the effect goes away. It seems the babies can't squeeze off the sides then, and they die along with the adults. So there is a work-around.