As you can tell from this tweet, villager zombies have some sort of secret. Let's try to find it!
We found it!
Using a Weakness Splash Potion followed by a golden apple on a villager zombie will revert them back to a normal villager! They emit red squiggles and, after about 5-10 minutes, will transform back into a villager.
I tired hitting one with a splash potion of weakness, then right-clicking with a golden apple, and the potion particles turned from red to gray. I also tried the same thing on a normal zombie and that didn't happen. We might be onto something here...