Drops from stone falling long distance next to fences, bounce up and down (frozen animation )

To recreate this dig a deep pit, (for me the pit was 15x60x15) surround th pit with fences, (I used oak) Mine 3-5 blocks over the pit right next to the fence. 

 

The drops from the stone blocks start to fall then glitch back up and continue this behavior until collected. Not sure if they eventually despawn