Did you check the return value of the OnDrop function to true after you’ve called the UseItemInSlot event? If you didn’t the game thinks this drop operation is unhandled and sends it up to the MainHud. This would explain the issue.
Don’t worry, I like to help. Those kind of variables and many others are read from the gamemode. There you have a nice overview of variables and you’re able to quickly edit them to your needs. If you change it there it will work.