You are unable to drop one item from inventory using x, but when you try to do that when looking in a chest it works without any issue.
How to reproduce:
- Open your inventory
- Take an item.
- Move the cursor away from the window.
- Press x.
What I observed: Nothing happened.
What I expected: You drop 1 of the item.
If you do the same thing, but open a chest instead, it works without any issue. This affects at least xbox one. I am really sure that it also affects ps4 and nintendo switch with the difference that you have to press a different button on ps4.