When you drag an item over another item stack of the same type it disappears until you stop click dragging it. The happens with all clicking types, middle, left, & right. It also happens in survival mode if you left or right click drag.

Steps to Reproduce

1. Get 2 stacks of the same item type
2. Drag one of the items over the other item of the same type
3. The item you dragged it over disappears until you release click