When going a quick click drag from one inventory UI element to another, it will pick up the wrong item, nothing at all and something the item that you clicked on. You are required to click and hold for longer until can see that items is visibly picked up before you can move it. It happens either the stash UI or the inventory UI.
Yip… Known issue thats been reported before, if I recall it was first mentioned in Feedback as it doesnt “break” anything really.
Seems to be more obvious to the millisecond First Person Shooter players or people who have their clickspeeds/mouse pointer movements set faster than normal.