Draggable re-appears at original placeholder position after beforeDrop accept

[arminh]

I have several draggable items and two lists that the items can be dragged into. The lists accept only certain items, which is is checked via the beforeDrop-callback.
When an item is accepted in the beforeDrop-callback and dropped into the list it sometimes happens that the item appears at its original position (I use placeholders) for a split second, just long enough to be noticable.

Here is a plunker to illustrate the problem: http://plnkr.co/edit/JuDUlnSDNu7VaA4PRoWi?p=preview
You may have to re-run it a couple of times since the bug doesn't appear all the time.

Do you have an explanation for this problem?

Thank you for your great work!