The bug

Closing your inventory while you are holding an item with your cursor does not drop it properly. Neither stat.drop is increased nor you are set as Thrower of the item.

Steps to reproduce

1. Create a stat.drop scoreboard objective and show it on the sidebar

   /scoreboard objectives add droppedItem stat.drop
   /scoreboard objectives setdisplay sidebar droppedItem
   

2. Put an item in your inventory
3. Switch to Survival or Adventure mode

   /gamemode survival
   

4. Open your inventory and pick up an item with your cursor
5. Close your inventory
   → The item is dropped but your droppedItem score does not increase