Scenario:
At the time of generation, the timesheet associates itself with the department of the user, any changes made to the users department  does not affect the timesheet that is already created.

Example:
Task A has Department Z assigned, User Y was in Department D but mid of the timesheet period the user was changed to Department Z.
In the above mentioned scenario, user Y will not see the Task A on the timesheet to enter time against it.
However when he generates a new timesheet the new department is associated with the timesheet and he will be able to see the Task A.