I put in some time to test the tasks feature as per the steps you mentioned and here are my test results:-
1. Assigned 2 tasks to a power user.
2. Logged in as the power user, finished the task for the reason it was assigned. All this while the 2 tasks were showing up in the "My Tasks" of the power user.
3. Finished the task and changed the status to complete for one task. After performing this step, I moved the file for which the 2nd task was assigned.
4. The task disappeared from the power user's list of "My Tasks". I moved the file back to its original location. The task had disappeared from the power users list. However, as an admin I was able to re-assign the task to the power user.
So from what I know, you should be able to re-assign the task unless there was a due date mentioned on the task.
If you do not see anything of the above happening with the tasks feature, I suggest you get in touch with our support team and seek assistance.
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