Mobs are despawning

i had a zombie villager in my cave house and i left to get a apple in the forest, (to make a golden apple 2 cure him) and i came back and the zombie villager was gone! i had the zombie villager in a part of the wall that was 1 by 3 so he could not have run away.

i later went on a test world and traped a naturally spawned (not with a spawn egg) zombie
and a unnaturally spawned (with spawn egg) in a 2-2-2 cobblestone box and and gave the unnaturally spawned 1 a pair of boots, I flew away (about 1-2 hundred blocks away and came back and sure enough the naturally spawned zombie had despawned.