Items (entity) sometimes floating on the sky and cannot be picked up

I was died accidently, and went back to the place where I was died. But when I arrived to the p;ace, I found that some items (entity form) were floating on the sky. It was more strange that when I approached to these items, I coundn't pick them up.

How to reproduce:

1. Just create a new default world.
2. Collect some items and let you die.
3. Go back to the place where your items has dropped.

Because the size my recorded video is more than 10MB, I can only support a screenshot.