I died a couple of times a few hundred blocks away from my respawn point and my items despawned even though regular items didn't and I got similar results in testing on another world.

steps to recreate:

1. Set a spawn point
2. Move a few hundred blocks away
3. Die
4. Go back to the place of death after the item despawn time

Another thing I noted was that when I died to a mob, it would pick up some items especially armour and weapons so I'd assume they're not instantly despawning.