So I found a mob spawner in my single player world .. then I build a mob spawner farm.
then I AFK, after 30 minutes AFK'ing, I got massive lag spikes. the chunk nearby the spawner is no longer playable since it's so lag that you got 1-3 fps.
Then I tried to travel 128 blocks away but it did not do anything
so I decided to travel 2048 blocks away to clear this mess, but when
I got home... I still got massive lag spikes... They just won't despawn.
now I got killed a creeper.
note: I tried the following solution.
Travelling 2048 blocks away from the spawner. Despawning failed.
Testing a new world: Despawning is a success.
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that's not a bug I'm pretty sure it supposes to work like this...

I'm not sure about that, when I tested it in another world it did work, they did despawn but in my world(It's on the pictures) they just won't despawn.
and I even read the Minecraft Wiki.
Duplicate of MC-151772