If you drag when you left click, you will drop when you let off the button over an inventory slot.
If you left click to drag and then left click to drop, what is happening is that you dropped it when you released and picked it back up with the next left click.
Try it. left click and hold, drag to slot and release the button it'll drop.
Not taking anyone's side here ,not saying that its fine or is garbage, just trying to save some frustration.