To drop an item on touch you have to hold the item in your hotbar for 1-2 seconds. When you try to drop an item in the 9th slot when using button size 100 (in my case I am using an iPad Air), the item will not drop. Instead you have to open your inventory and throw it manually.

Steps to reproduce:

1. Have an item in the 9th slot.
2. Try to hold the item in the 9th slot to drop the item.
3. Item isn't thrown.

Expected results:
The item should be thrown

Observed results:
Item stays in the slot.