Due to the default "long" NumberAnimation duration of 250ms, the drag and drop example and therefore the drag and drop functionality, in general, can end up in a faulty state.
If the user moves the dragged Rectangle fast enough (quicker than the transition) on top of another one it will stay in this faulty state as shown in the video. This can be a workaround by setting a smaller duration time on the Transitions NumberAnimation and can be completely avoided by setting it to 0.