Note: This issue is also reported for Java Edition at MC-175294. There it seems to be a mere annoyance rather than a farm-killer because of the differences in spawning mechanics between the editions.

Despawn-immune monsters can accumulate in areas the player has never visited and cannot easily find, such as hidden caves and small surface pools. Over time they fill the monster population caps and prevent further spawns. This is despite the despawning changes in 1.16.0 that were meant to fix this problem (MCPE-21856, MCPE-34032). The cause of the issue now is the fact that mobs become persistent when they pick up items. This can happen without player interaction. In particular:

• Monsters in hidden caves can drown or be burned by lava and drop items, which are then picked up by zombies.
• Monsters on the surface can burn or fight each other (e.g. if skeletons or trident-wielding drowned attack naturally-spawned iron golems, or trident-wielding drowned attack villagers, and accidently hit another mob, starting a fight), or be killed by naturally-spawned iron golems, and drop items which are then picked up by zombies.
• A zombie can spawn with armor and the ability to pick up items, walk into a pool, drop its armor when it drowns (MCPE-33140), and immediately pick up its armor again (MCPE-89100), becoming a persistent drowned. When a surface-cap zombie drowns it becomes a cave-cap drowned (MCPE-61721), allowing an infinite number of drowned to build up (MCPE-34032). It is also possible for skeletons or other drowned to pick up armor dropped by drowning zombies.

Steps to reproduce

1. Create an infinite world, creative mode, with difficulty set to "hard".
2. Afk for 24 hrs somewhere in a non-ocean biome the overworld where there are caves and 2-deep pools.
3. /tp ~ 255 ~ to despawn monsters.
4. /execute @e[type = !player] ~~~ say hi

Expected results

"No targets match selector."

Actual results

[Zombie] "hi"
[Drowned] "hi"