My experience is similar. Shortly after the latest minecraft update, my zombie grinder didn't work like before. I had a 20+ block elevator where they would rise and then drop to the kill zone. When the zombies approached the top they would all disappear. When i went to the top to inspect what was going on, they stopped disappearing and the grinder worked as before. I think something may have changed in the latest minecraft update that affected the distance you could be from spawned mobs. At any rate, i "fixed" my grinder by shortening the height of the elevator. I believe the 16 to 20 block distance others have mentioned holds merit. Although it's not as fun as before and the zombies take more hits before dying, there is a work around.
That's my two cents!
Max