Items dropped by the player directly from the inventory menu will despawn after only 1 minute. Items in Minecraft typically despawn after 5 minutes so I believe this to be a bug.

To reproduce:

1. Throw an item on the ground by dropping an item from the inventory menu (NOT by pressing the drop key)
2. Observe the item despawns after 1 minute

 The bug can be seen in this short video here: https://www.youtube.com/watch?v=ynmw3S2mLHM&feature=youtu.be