When creating a timesheet using the add assigned task timesheet feature in the PPM/Clarity New User Experience (New UX), tasks are able to be added even if the task dates are not within the date range of the timesheet if there is ETC left on the task and the assignment finish date is prior to the timesheet time period.
This is by design. The application will adds tasks with non-zero ETC in time periods that occur and after the assignment finish date. Tasks will also be added using the Add Assigned tasks if they have a date range within the time period of the new timesheet (or fall within the Populate time range set in your Administration->Timesheet Options).
The tasks can be removed by the user after timesheet population (or the ETCs can be removed to prevent the tasks from showing up outside the date window).