Dropped entities not floating in water

I was mining sand in a small pond on multiplayer, and noticed that only about half of the tile drops were floating to the top of the water. I asked a moderator if we have a plugin that prevents this, and he said no. I repeated the action a few times, and only once did the entity successfully float to the top of the water.

To get the bug:

1. Drop an item in water at least two blocks deep
2. Observe