Here are the steps to reproduce the issue:

a. Navigate to a Card Wall populated with stories and tasks -- and more than one tasks for a story in a column (i.e. Not started)

b. Hover the mouse over a task and when the mouse changes to a movement ‘cross’, click on the task and drag it only by a few pixels in any direction.

c. The task that was just moved by a few pixels will jump to the bottom of the tasks listed in the story…

Note: that a card has “jumped” once with the above steps, it will not ‘jump’ again