Hostile mobs do not despawn, even after I've walked hundreds of blocks away from them. There are mobs that spawned on my first days on the map near the world spawn that are still there after over 30 hours of play thousands of blocks away. Visibly reduced mob spawning at night due to the ammount of drowned on nearby chunks (sometimes 20 or 30 inside a single water hole).
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Auldrick
migrated
Thank you for your report!
We're actually already tracking this issue at MCPE-21856, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Note: Although the description of MCPE-21856 only talks about spawning, spawning and despawning are both parts of the same balance algorithm and despawning has been extensively discussed in the comments.
You may also want to review the wiki article on Spawning for information about how spawning and despawning algorithms differ between Java and Bedrock editions of the game.