[MCPE-21416] Mobs and Entities can despawn randomly

Update November 2021

We are tracking an issue with all or nearly all mobs in the area around the player vanishing when the world is reloaded in versions 1.17.30 through 1.18.2 Hotfix at ~~MCPE-144208~~. Please do not comment here if that describes your experience.

Moderator Note by [MCPE Mod] Dr.Awesome4333 and [Mod] GoldenHelmet

Please limit comments to New information about the issue. A short list of what is most helpful and what is not is listed below. Please take the time to read this brief list before commenting.

A list of mobs you have lost is not helpful information. We know this bug can impact any mob, we know that leashed mobs that despawn will leave behind a leash knot, and we know that when villagers despawn their beds and workstations remain linked to the missing villager until you break them or 20 minutes pass. What might be helpful are details about things like

what you were doing leading up to the mobs disappearing
single vs. multiplayer, local vs server/realm
how far you traveled
how long the current or previous play session lasted
whether the despawns were near a player spawn point or if you had changed your player spawn point during the session,
if you experienced lag or anything else abnormal.

Workaround (updated Dec. 5, 2020)

Some cases of persistent mobs despawning can be traced to them crossing chunk borders between saves, so one way to guard against randomly losing mobs is to prevent them from crossing chunk borders (you can read about finding chunk borders on the wiki). However, this is not a guarantee or a fix. Positioning mobs away from chunk borders does not mean you will never lose a persistent mob again. Many cases of mobs randomly despawning remain unexplained by the tracker staff and player community.

Updated description by [Mod] GoldenHelmet July 22, 2020
This ticket is used as parent for all otherwise unexplained mob disappearances. It covers mobs disappearing related to nether portal travel, long distance single-dimension travel, crashes, realm lag, and any other circumstance, except for the specific circumstances covered by ~~MCPE-88322~~, ~~MCPE-66818~~, MCPE-51837, MCPE-16863, and ~~MCPE-108568~~, ~~MCPE-141539~~, and ~~MCPE-144208~~.

Some random mob despawns can be explained by the mobs crossing chunk borders between auto-saves. Other random mob despawns are reported for mobs that were unable to cross chunk borders.

I have attached a test world that can be used to reproduce despawning on chunk borders due to a crash with the following steps to reproduce:

Load [EX] Chunk border despawn demo.mcworld.
Press the button to spawn a villager.
Save & quit, then reload, to confirm that the the villager is kept in the world save.
Open the world, flip the lever, and start a stopwatch.
After about 10-15 seconds, close Minecraft without exiting the world (for example, in Windows 10 press alt-F4).
Reopen Minecraft and reopen the test world.

Expected result

The villager is still present.

Actual result

Sometimes the villager will be gone. I estimate this occurs about once out of 20 attempts. You can see the entire sequence in Chunk border despawn demo.mp4.

What happens when the villager disappears is that sometime between flipping the lever and force-closing the game, the chunk in which the player is located gets auto-saved and the chunk to the right does not. You can verify this by observing that a honey block is also missing--the fact it was pushed out of the left chunk by the piston is saved, but the fact it was pushed into the right chunk is not.

Original description
When I go to Nether and I went back my horses an my animal that I feed is disappear

Edit by Zeb: When playing on multiplayer, entities can despawn when you travel to the Nether and back to the Overworld.

Keep in mind that naturally-spawned monsters and animals that have never been interacted with are SUPPOSED to despawn. When testing this bug, please make sure to interact with monsters or animals you're observing first, by tempting them with food, breeding, or name-tagging them. This is not necessary for villagers or iron golems.