When we need to move a task in multiple lists from one list and keep the other list assignments intact, the process I have found to work is to 'Add' to the other list, then 'Remove' from the current list. Otherwise, all currently assigned lists are cleared. Ideally, would like to Move from only one of the assigned lists and leave the other assignments as-is. However, it would be helpful if, when I select Move for a task with multiple lists, I am warned that I will clear all the current lists. When performing a group move it is easy to accidentally Move instead of Add/Remove since Move/Add are the same option on the popup toolbar, 'Move' is the default tab at the top, and the selections for each look the same. When this happens it's a time-consuming process to put our list structure back since it needs to be done on a task-by-task basis. It would also help to separate Move/Add on the toolbar so it's not a two-step process to Add.