At the moment a treeview always clears the selection when the user wants to initiate a drag. This happens if the selectionMode is set to ExtendedSelection and the default drop action is MoveAction. The user presses the Ctrl key (to copy) and then pressed on the selected item.
I assume that it's interpreted as a selection change (clicking on the selected item while Ctrl key is pressed deselects it). In contrast to e.g. the windows explorer Qt performs selections of items while the Ctrl key is pressed on mouse press (explorer performs it on release or when the drag is started.
Even if it sounds like a minor issue it's quite annoying to take pressing the Ctrl after the drag was initiated.

You can reproduce the behavior with the attached .ui.