On a 1.9.2 server I have this bug where all hostile mobs in a dimension despawn as soon as I enter this dimension. I found out about this when I was building my guardian XP farm. As you can see in the video not only the guardians despawn, naturally spawned mobs also despawn. I also went to the nether with an other portal which wasn't in reach of the chunks of my "guardian portal", but they still despawned.
Then I spawned a Creeper in the Nether and pushed it through the portal. When I went back to the overworld it wasn't there anymore.

Because there are many mob farm designs with a portal like mine I think this has to be unintended. The world from the server also works fine in single player, so this has to be multiplayer/server bug.

Link to the video: https://youtu.be/dXv8AXMKe7Y

Yours sincerely
diabolo66