I recently built a box to breed villagers in. It is a 13x13 area with 12 beds. I noticed that when I went to check on them 1 adult villager and 2 baby villagers were missing. Does anyone know what could have happened? There is no way for them to escape or for zombies to get in. (I play on 1.14 pocket edition).
From this image it looks like the door is open so they could have wondered out. If they wondered out without you noticing at night then they could have been killed by mobs so you couldn't find them.
This is a link of one good villager breeder that you could use but there are plenty designs if you google villager breeders. Iron Golem farms are another source of these villager breeders because the villager breeding is the first step so you can look there as well.