I have a system to automate villager to zombie villager conversions, and to do this I figured out that when in a minecart, a zombie villager is slightly shorter than a villager, so if I put it on a half slab, the villager will trigger the a string next to an observer while a zombie villager will be below the string, unpowering it. Or at least it should. I tested both the villager and the zombie villager separately, and the above thesis was confirmed, but when I put it into practice it didn't work. I put a villager in a minecart in a chamber next to a zombie that could convert it into a zombie villager. I made sure that the zombie cannot trip the string by lowering it by a half block. When the zombie converted the villager into a zombie villager, the hitbox went below the string but the string didn't update to become unpowered like it should have. I tried keeping the zombie villager in place but replacing the string, and what do you know? The new string was unpowered. As is visible in the screenshot, the zombie villager's hitbox is well below where the string is.