Items sit on top of hoppers instead of going in them

In certain instances (such as killing a mob on top of a hopper), a drop/item (such as a bone or arrow), will not go into the hopper but instead sit on top of it, and cannot be retrieved unless the hopper is walked on or near (as in a block diagonal to it). In the instance of killing a mob on top of a hopper, experience orbs on occasion will stick in the lid of the hopper if the player is standing at eye level to it, instead of gliding on the top of the hopper and to the player.

One of the cases is if the hopper was locked then an item was on top of it, then the hopper was unlocked. Video: https://www.youtube.com/watch?v=776QfTz7j1U&feature=youtu.be