While travelling in a boat I came across a skeleton standing in water on a small desert island in the ocean.  I thought nothing of it until I traveled several hundred blocks away and came back 10+ minutes later.  It appeared the same skeleton was standing in the same spot on the same island. 

I started testing various distances trying to get the skeleton to despawn starting with the normal 128 blocks away. I came back and it was still there.  I also tried relogging and the skeleton was still there.  I continued the test going out 150+, 200+ 250+, 300+ and 320+. At each distance the skeleton was still on the island when i returned.  

Throughout my 5+ hours of playing I consistently noticed a lack of mobs in the world.  I believe this is because mobs are stuck somewhere filling the mob cap and not despawning.