Mobs despawn faster than before

Before the 1.16 update, if hostile mobs (like zombies or skeletons) were in confined area, they would not despawn until killed. After the 1.16 update, if I trap a mob and then leave the chunk, they automatically despawn. For example, I wanted to turn a zombie villager into a normal villager. I trap the zombie villager, run about 30 - 40 blocks to my house to grab a golden apple, and then come back, the zombie villager despawns. This happens to all mobs including passive mobs if they are not in a confined area. But why does hostile mobs now despawn if I go about 20 blocks from them even when they are in a confined area?