We recommend AFKing 48 blocks above the lowest platform. To prevent unnecessary mobs from being loaded in caves, the despawn radius is a lot lower than normal Minecraft. The despawn radius is as followed:

• Soft radius: 24
• Hard radius: 72

At 24 blocks away, mobs will start to spawn.

At 72 blocks away, mobs will despawn instantly, so make sure the mobs are hitting the ground and not despawning.