Dragging a task from goals list to a weekday - dropped when changing the week

Here's another bug, unfortunately ... when I create a task in the goals list, drag it to a weekday and then change the view to the next week (>>), when I change back to the current week, the regarding task has been deleted from the weekday (but is still in the goal list) - is this a bug or a feature?

Cheers
Dennis

Browser: Mozilla Firefox 38 on Xubuntu (Ubuntu 14.04)

EDIT: Strangely. when doing as described, and a time xx is set for the task, the time is still left in the weekday (e.g. xx minutes left)